Whining/spooling/spinning sound on deceleration

[xsports33]

I'm getting this very weird loud almost spooling like sound (whining, spinning sound) when i'm decelerating or taking my foot of the gas. it happens in all gears and at all speeds. it's almost like when my engine goes into a "deep" vacuum it starts making that spooling/spinning sound. I've checked all the connections, nothing is loose, no CEL, nothing obvious under the hood or under the truck. Specs of the truck in my sig

it happened when i was coasting down a mountain, no foot on the gas. i was in vacuum for a couple seconds, then all of a sudden i started to hear a loud spooling/spinning sound coming from the engine compartment. this has never happened before, and i've never experienced it in any of my vehicles.

any help would be great.i'll try to paste a link to a video i captured when it happens. you can watch the RPM meter drop, then all of a sudden you hear the sound. If i give it a little gas, it goes away immediately, pause a second and it comes back on decelerate.you can hear it from seconds 7-11 then 13- 16, and again from 22-24.

here's the link: https://youtu.be/at3Hqz9ogtI

thanks in advance

 

[Virtus_Probi]

I notice something like this when it's very cold out and my engine hasn't warmed up yet (which can take quite a while at such times, even on the highway).
Did you notice this in very cold conditions?

BTW, you must be on a trip...no mountains in the greater Chicagoland area or anywhere nearby, unless you want to count Mt Trashmore in Evanston. I spent many a happy day sledding there as a kid...the only other places to sled nearby were highway overpasses.

 

[MrHorspwer]

I'm not seeing any signature, but it appears to be a GMT800 truck of some type, probably a 2500 or 3500 based on the trans temp gauge on the cluster?

 

[xsports33]

sorry.. i thought my signature line was active. here's the stats:

2006 Ext Cab Silverado 5.3L HO 4x4 * Radix * 3.2 Pulley * 90mm TB/J-Tube/Dr. X * HPTuners * Autometer Gauges - Wideband, Fuel Pressure, Boost * Volant CAI w/Ram tube * Pacesetter LT and ORY * Magnaflow Dual Rear Exhaust * Magnaflow 5" Resonator * 6" RCD Lift * 35" M/T ATZ * AR Victor Teflons 17x8 * 4.56 Gears * Mag-Hytec Diff & Trans Cover * TruCool 40K * Ride-Rite Air Support * FLT Lvl 5 w/300M shafts

I lived in Chicago about 6 years ago, then to San Diego, now i'm back in Maui, HI.

It was about 65* when this happened. so no, not cold. but i've never experienced this before. it literally just happened all of a sudden while cruising down a mountain.

 

[spasm3]

Hard to say not hearing it in person. Could it be some sort of heat shield vibration from exhaust resonation? I'd check and see if any heat shields are loose.

 

[MrMoody]

Sounds like transmission or maybe transfer case grinding ... not good. Drop/drain the pan, look for metal.

 

[CBR.worm]

It may be an odd vacuum noise - I don't know what the PCV system is like, but I've heard some PCV valve buzz or whine. Not that loudly I wouldn't think, but who knows.

 

[sloinker]

I had something like that a few decades ago. I'll throw it out there as it's easy to check even though it's doubtful your vehicle even has this. The fan clutch made a weird whirring noise on deceleration. Replaced it and the noise disappeared.

 

[Warstud]

Originally Posted by sloinker
The fan clutch made a weird whirring noise on deceleration. Replaced it and the noise disappeared.


That's what I was thinking.

 

[xsports33]

No fan clutch - i only have electronic fans

if it was the transmission or T-Case - why would it only happen in deceleration when my engine is in vacuum? wouldn't it happen all the time? it really sounds like it's coming from the engine compartment, not underneath. I'll check anyway though

i think it's vacuum or fuel related - but i'm running out of ideas on how to check that. I'll check the PCV.

 

[clinebarger]

You have a Magnusun Radix Supercharger? Check the vacuum line to the bypass valve & bypass valve operation itself.

No PCV valve on a 2006, Metered Orifice in the left valve cover.

I'm guessing you know the pump issue that plagues the New Venture T-cases in the GMT800 platform? I would at least check the fluid level!!

Also make sure the belt tensioner isn't bottomed against the stop, Belt too long, Worn belt.

 

[xsports33]

Originally Posted by clinebarger
You have a Magnusun Radix Supercharger? Check the vacuum line to the bypass valve & bypass valve operation itself.

No PCV valve on a 2006, Metered Orifice in the left valve cover.

I'm guessing you know the pump issue that plagues the New Venture T-cases in the GMT800 platform? I would at least check the fluid level!!

Also make sure the belt tensioner isn't bottomed against the stop, Belt too long, Worn belt.



THAT would make a lot of sense!! i'll contact Magnuson tomorrow to see about a new bypass valve. thank you!

Any ideas how to check the bypass valve manually? there's a very large "do not adjust" sticker on it. i'm well out of warranty. but the truck is "driveable" now, and i don't want to lose that.

i'll check the best tensioner as well.

any other ideas?

 

[clinebarger]

Have someone start the engine while observing the valve actuator/arm, The arm should move all the way forward toward the front of the truck.

You can also check it with a hand operated vacuum pump, 15 inches of vacuum should be enough to fully actuate the arm. And should hold vacuum 'til you release it!

I wouldn't buy a bypass 'til you confirm it's faulty!

 

[WyrTwister]

Any chance ithe noise is coming from the rear differental ?

 

[xsports33]

Thanks, clinecharger, for the feedback. I contacted Magnuson and they walked me through testing the bypass actuator, turns out that is most likely the problem.

the loud spooling/spinning in decelerate is actually the supercharger rotors spinning faster and faster from all the air going through from a stuck open bypass valve. In vacuum the bypass valve is supposed to close allowing the rotors to free spin, but in my case that is not what's happening.

if you're ever in Maui, i'll buy you a couple rounds.

 

[xsports33]

So it turns out its NOT the bypass valve. $200 later the problem still exists. So Magnuson's theory of the supercharger rotors free spinning is incorrect.

however, i have narrowed it down to the drivetrain. I thought, shot in the dark, i'll put truck in 4WD and see what happens. Well, the sound went away completely in deceleration when the truck is in 4WD. i put it back in 2WD and the sound came back.

any new thoughts? maybe the front differential, wheel bearings (new though), transfer case, i hope it's not the transmission. i didn't have time tonight to look at the fluids for any, but i think tomorrow i will be draining all of them and look for metal?

 

[mehullica]

Sounds like a bad pinion bearing on the rear differential

 

[MrMoody]

With it changing 2WD vs 4WD, I'm in for transfer case.

 

[xsports33]

i haven't checked the fluids yet, but i found that where the front driveshaft meets the transfer case there is a little bit of play. not much, but a little. none at any of the other connection points.

if it is a pinion bearing will it leave metal debris in the fluid?

 

[WyrTwister]

Forgive me , I missed how many miles are on the truck ?