Rumbling after rebuilt transmission

The transmission died on our 2003 HOnda Odyssey (less than 70K miles) and we had a local mechanic put in a rebuilt transmission a couple months ago. Ever since, I have noticed a rattling noise in the engine that only occurs when the car is going about 1500-2000 (less than 2000) RPM and usually happens around 45 MPH. It was difficult to reproduce at first but lately I am noticing it pretty consistently at 45-50 MPH when I’m just slightly accelerating. THe rattling stops after a few seconds. The mechanic said it sounded like a heat shield, but doesn’t make sense that it doesn’t rattle all the time. Then he put the car up and banged on the pipe going into the catalytic converter and heard some rattling and thought that might be it. So I took it to a muffler mechanic he referred but that guy checked out the catalytic converter (visually) and the pipes that go into it and said it’s fine. (I also took it to the dealer and they told me I needed a new B pipe- costing $500 - but I don’t believe them.) I had another mechanic look at the cat too and he said it’s fine. So I went back to the mechanic that did the transmission and he said not to worry about - it’s probably just the “heat shield.” It seems transmission-related to me. I have also noticed a louder “rumble” going at lower speeds (25-30 MPH) only when I’m slightly accelerating. This is similar to the rattling noise because it only happens when I’m slightly accelerating, which makes me think it’s the transmission. Any ideas?

Do something to the heat shield so that it cannot rattle. brace it, remove it, or clamp it in place so that it cannot rattle. If the rattle goes away, then it was the heat shield. if the rattle is there when the heat shield cannot rattle then, the rattle is somewhere else.

Did you take your Odyssey to a dealer shop?

These things have been plagued by transmission issues… they might have helped as a goodwill repair…

In the past some shops had rollers with bumps on so that you could duplicate road conditions to look for rattles. I think this was in the old days whan almost all cars rattled. There might be some of these around. If the car is not well tied down they can be dangerous.