Listen for flies. If you hear flies on the other side of the mud pile, that means the room you're about to enter contains a body pile. The body pile will keep spawning Draugr until you take it out, and it could spawn a Draugr Elite, so you need to take it out ASAP.

I find that the safest way to take it out is to cut a path to it as safely as possible and take it out with a bow, then take out the remaining Draugr. Sometimes that's not easy, but it's okay to just forget about it and go someplace else. Adding your body to the pile is not recommended.