I've had my truck about a year (93 150 300 I) when ever you turn the heat, ac, defrost, etc. on it makes a thump thump thump sound, until you are going highway speeds or turn the fan up on high. I haven't worried about it much because I had other things to fix and the heat, ac, defrost all worked fine, just made a little noise, any ideas on what this could be, how to fix, and how much it might cost is appreciated.

 

Maybe the bearings are shot in the blower motor. This time of year I see alot mice have moved indoors...possibly into your blower. It doesn't take much for those to become out of balance.

 

possible mud daubbers nest in there too .

 

this may be silly, but can you get in there to look and clean it out, maybe shoot some lubricant on them bearings???