I have a 2003 H2 with 55,000 miles. I bought it 6 months and about three months ago I noticed the engine knocking. It's not just at startup. Its a low idle knock. Idle to 1500 rpm then goes away. I have tried several engine oil additives but nothing has worked. A mechanic told me piston slap, but this is low in the back near the cabin on both sides. I read a couple of posts about some bolts being broken and the problem was exhaust leak. Can anybody give advice. The truck runs good, and I don't want to pay mechanics to experiment on this problem. I'm debating replacing the motor or rebuilding, or something. Please help.