knocking trans

[shomanv6]

I just recently had a midwest y-pipe and new 02 sensors put in my car by my local mechanic:who I would like to run my car threw his shop any way when I got my car back at low rpm in first and second gear if you let off the throttle it will knock which feels like it is coming threw the shifter and in reverse when you let the cluch out it knocks constantly what would cause this If any sho experts could chime in on this it would be helpful

 

[shomanv6]

fixed the problem:the bolts that came with the y-pipe were to long and banging agaist the shifter cut the bolts and know sounds fine.