99 Yukon Knocking?

[bramas02]

About a week ago I noticed that when I accelerate hard from a stop my Yukon will tick. I don't here anything at idle or if I take of slowly. I have a few of my own idea's what It could be but I was hopping I could get some idea's from people here.

[bramas02]

Well one of the things I was thinking was that one of the cats where pluged and this morning it finally put out a code P0420 (Catalyst System Efficiency Below Threshold (Bank 1). So could this be my problem?

[GM-Tech]

Sounds like you have a bad catalyst. Whether or not that's the cause of your other problem is questionable.

[ManyCamaroz]

Your title states "knocking" and your description states "ticking". Two very different sounds.

 

Which is it?