1997 GMC Suburban Musty smell

I recently purchased this vehicle. I have a musty smell inside my vehicle, with the vents on and or heater and a/c on, smell is about the same. This smell reminds me of having a dust leak on a gravel road. It actually plugs the sinuses after driving for a little while. I checked to see if I had any noticeable leaks and I can't find anything. I wonder if there is a in cab air filter that needs to be replaced. Some people I have talked to say it is sounds like a mold problem. It doesn't smell like mold to me. The oder smells like really old oil--not burning oil, just old oil. The engine looks clean no oil leaks. Please let me know if anybody has had this problem before.