hello i have a 00 PSD, today i cut off the muffler, while i drove around in like 2nd gear i could hear this noise that sounds like air beeing sucked in somewhere. the sound was there before i took the muffler off, i also asked a buddy of mine that has a psd and he said he doesnt know what the sound is but that all psds make the noise. can someone please tell me what it is? thanks

 

also where can i get a 4inch down pipe really really cheap, im kinda low on money right now. thanks