i had a similar sound that was the crossmember touching the exhaust at certain rpms. diconnected the crossmember bolts and put a washer on the top side of each bolt. probably not what you have if you think its in the dash.
02 Tiag e46 M3|6MT|GC plates|MCS c.o.|GC bars|GC race control arms|GC bushings & bearings|BW eng. & tran. mounts|subframe kit|BW race shifter|BW Jaffster|Euro header|BW race|K&N c.a.i.|Epic race tune|Rouge race pulleys|Seibon CF hood|CSL bumper|ST-40|SR34|SS lines|half cage|Recaro profi|Profi 2 harness|BMWpedals|Apex studs|