i like the cranked up stereo idea
same noise in neutral ? when u rev the throtle pulley a little does it change ? with ac on /off ?
spray little of wd40 on the belt ... see if it goes away .
hmm cant think of anything else ...
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84 928 , grrr, love and hate
87' 924s sold

88' 944 track project
951 R.I.P. it was a great friendship