2000 GMC Yukon Rattling

My 2000 GMC Yukon Denali sounds like a diesel truck knocking and rattling while in park and driving at low and high speeds. The car runs like a top, nice and smooth, at 75 mph on the interstate but still makes these sounds. I drive in a lot of mud while hunting and fishing, so it sounds like maybe the pulleys and belts are dirty. I looked under the hood while it was in park and running and the sounds are not coming from the engine itself but from just in front where the belts and pulleys are. Could it be that the belts and pulleys or a bearing of some sort need cleaning or changing? Thanks for your input.