5th Gear Decel Whine/Vibration/Rattle/Whatever you want to call it
 

Well, what I "think" is a known thing I just simply want to confirm with the million of variables I have going on here.

Just got done with a rebuild/replacement of a stock FA20. New engine is an IAG 600, GSC Valve Springs, GSC Valves, Harrop SC 90mm Pulley, ID1050x, etc etc.

Prior, I had a stock clutch, flywheel, pressure plate. Now, I have a clutchmasters fx350 with their steel flywheel...which may be lighter? Not entirely sure - it's not scalloped or anything to the extent of other flywheels. But we'll consider it lightweight.

Took the car out for a first test drive, after some stumbling and issues maintaining idle which what we think is related to my PTuning MAP sensor (tl;dr car threw a P0107 - Manifold Absolute Pressure/Barometric Pressure Circuit Low Input. We swapped back to an older tune, OEM MAP and the CEL went away).

ANYWAYS - I found the flywheel chatter, NVH while lightly accelerating, definitely some resonance occurring under load which is to be expected. Nothing out of the norm. However, whatever reason trying to downshift from 6th to 5th produces the most violent NVH compared to any other gear while decelerating. Definitely think it's related to the Flywheel/Clutch assembly as if I tip-in minimal throttle the noise goes away, lift off throttle in 5th and the noise comes back.

I do have whiteline diff bushings (the inserts and new outrigger bushings) but that type of noise wasn't present prior...though also remember - I had a stock clutch and flywheel.


Lots of ramble, and while I tried to get it on video truthfully the resonance is so deep that it doesn't really appear in video. What I initially thought could be exhaust related (like cam phasing or something related to VVT) is more under the impression it's drivetrain.

The plan is to check motor mount torque (STi Mounts) and the Transmission mounts (OEM) and see how those look. When the transmission came out we didn't separate the carrier so don't think it could be the transmission mounts but trying to see what else I can look at.

Thoughts? TL;DR - I think I'm hitting a weird resonance on decel ONLY in 5th gear that somehow is related to my flywheel and clutch and/or diff bushings that have been existing and made no noise prior

A light flywheel with STi mounts will definitely give you decel noises. I am just bolt on NA including a TODA lighweight flywheel. I eneded up going back to stock mounts to reduce the decel noise.