My friend has a 98 DX and he found a crack in his exhaust manifold the other day. He is about a week away from taking a 5 hour trip to look at the law school he is attending in fall. He ordered a stock replacement, and after putting it on, he says he is getting a bumble bee type sound at certain RPMs.

I asked him to check his gaskets and he said they were all new and that everything was tightened down. The one thing throwing me off is why he is getting the sound at only certain RPMs. I have not personally heard it as I live 2 hours away from him, but I was just curious if anyone here had ever heard of such an issue and could offer some advice.