When it is raining and I start the engine and put it in Drive or Neutral, there is a loud rattling noise behind the engine near the exhaust manifold. As the engine warms up the noise goes away within 5 minutes.

Sounds like it is either from the engine, or the exhaust line, unsure of which as I am in the car when it happens.

My dad and I don't believe it is a belt, but are unsure of this issue.

Any suggestions or ideas?