When the van is in park and idling the engine runs very smoothly. When I put it in Park or Drive, the engine makes a weird sound. i wish I could describe it better, but it makes a womp/womp/womp sound. Put it back in Park and the noise goes away.

Could this be a transmission problem that I'm actually hearing, when you load up the engine/transmission? Other than occasionally not shifting properly (once per month, maybe), it drives fine. The fluid was changed about 20K miles ago. Vehicle has about 85K miles on it.

Any help is greatly appreciated