I've noticed this a lot with my car lately especially when waiting at a red light or drive-thru for example.
If I am idling in gear with my foot on the brake it sounds like the engine is about to blow up but it smooths right out and sounds perfectly fine if I throw it into neutral or just park it. I checked the transmission fluid level and everything else while I was in there, everything was where it should be.
Video of the sound, ignore the SnapChat caption because I took the video before I realized the sound goes away if I put it out of gear:https://www.youtube.com/watch?v=n7YeL3Phep4
Again, it only sounds like this if I idle in gear with my foot on the brake, but sounds normal if out of gear.