So I replaced a rear brake line in my 98 Chevy truck a few months ago and with some help here on the forum got the air out of the lines. I also replaced the front brakes, calipers and rotors a week after. It has now recently started making some noises that concern me. I only drive the truck maybe once or twice a week but today it sounded like something was vibrating/rattling under the floor of the truck. It is an extended cab and my daughter said she could feel it at her feet. It would stop when I stepped on the brakes but soon start again. It only does it when driving but not ALL the time while driving. Brake pedal feel has always been slushy on this truck and brakes seem to work the same. I tested the booster with several pedal tests and it seems to be airtight. A few weeks ago while idling I thought I heard some whistling almost like a vacuum leak but couldn't be certain. It is hard to diagnose since only hearing it while driving but it is loud enough to know that something is not right at the truck. Maybe need to take it to a real mechanic but would like to diagnose it and see if I can repair it myself. Any suggestions on where to start?