Help tracking a metallic rattle...
 

Was hoping you all could help me brainstorm to figure out where/how to look for this metallic rattle I've been experiencing for a couple years now. It appears to happen in the 3000-3500 RPM range, is loudest on engine braking in gear, but seems to also happen during accel. So anytime there's load, I guess.

This rattle has persisted through a new clutch/pp/flywheel install, a swap to a '99 transmission, and upgrade to the 1.8L driveshaft and differential.

So far, the only thing I've tried is tapping around the tranny/PPF (because that's where it sounds like it's coming from, especially with the inner shift boot removed!) area with a rubber mallet, and the only thing I noticed was the speedometer cable seemed to rattle a bit, but it's not really a metallic noise. I tie-wrapped it in a different position for the hell of it, but it didn't make a difference.

My exhaust is all home-brew from when I put the turbo on the car a few years ago, but it seems to have enough clearance in all directions, and the stock heatshields have long since been removed. Plus, if the exhaust were rubbing, it would be more of a constant drone/buzz/rattle that likely got louder with RPM, whereas this seems to happen in a window of RPM.

Any thoughts or ideas?

flapper valve rattling on the exhaust housing... tighten down your wastegate rod a bit and see if that helps. I had the same problem for a while.

Hmmm, that's an interesting theory. I don't have a threaded wastegate rod, but I could try shimming it I suppose.

Last I checked, though, it felt pretty tightly-seated at rest.

+1 on the WG rod/linkage thingy. Mine used to do that.

Exhaust hitting the rear axle?