Rattling when idling in D after mud

Last weekend I was in Death Valley and after all of the recent rain there was a ton of mud, puddles, and a few shallow river crossings. Aside from all the mud/water there was just a lot of washboarded/rough road. Midway through the trip I noticed a rattling/rumbling sound coming from what seems to be the engine compartment. The sound happens when in drive with the brake pressed (stopped) and also seems to rumble a little while at low RPM's during acceleration. Any ideas for what could be the issue or suggestions for what to check? No check engine light, drove home (~200 miles) with no issues. 2009 4x4 V6

 

Any ideas guys? Going to spend some time looking at it this weekend but don't know where to start. No vibration/rattling when in Neutral or Park, but when I put it in Drive and let it idle it sounds pretty bad.

 

I have a rattle like that as well that's driving me nuts at the moment. Only happens when in D or R and the drivetrain is "loaded". My first thought was that it was the intake, however my worst fear is that it's an engine mount. I can't replicate it in Park, so at the moment I don't have anyone that can sit in the seat and hold the brakes for me.

 

My truck started doing the same thing this week. It only rattles from the passenger side in Drive or reverse. I wondered if it had something to do with the cold weather.. did you ever solve the problem?

 

No not yet, I've been neglecting the truck until I get another car out of the garage.