knocking noise, have idea but need opinions

[Tom]

Ok Ive had this knocking noise for a while now. I think I have it pin pointed, but Im not sure. With my ramhorn manifolds and 2.5" exaust tubes, the tubes come extremly close to the cross memeber/frame up front. I believe the knocking is the tube on one side hitting the frame. I came to this conclusion because it only happens when at a stop or slowing down. If I hit the gas, it goes away. I think when I hit the gas it torques the engine over and either puts the tube in constant contact or it moves to too far away to make contact. Also if Im at a stop or slowing down and give it a little gas the knocking will go away.

The other possability Im thinking is a bent flexplate/starter or somthing along those lines. The knocking only happens after the truck has warmed up a bit. Could be from the tubes expanding, or a flexplate. What do you guys think?