OK, I've got a vibration that comes in at 3.5k rpm, then goes away if I hit 4k rpm. it doesn't do it if i'm floored, only when cruising at 3.5k. any ideas?

I'm running 8 pounds boost on a 96 miata. I've got flyin' miata computers, and intercooler, BOV, ram style header and a turbo taken from a 89 thunderbird. the vibration is so bad that the bolts holding the turbo on continually come loose, and two headers have cracked. it's not the exhaust, it's not hitting anything.

HELP!

 

One more note- i feel it most through the pedals. and it acts the same whether i'm sitting still, or on the freeway. it's rpm based, not speed based.

 

check your tires

 

wait, you're running a turbo and a header?

Also, does it make this sound/vibration when you're in neutral?