Was today. Went from Stevi to Kalispell and back. about 1hr flight each way. Coming back I usually get Flight Following due to the airliners coming into MSO from all directions. So im at 9,500' and using Flight Following. I'm scanning for traffic, got the local CTAF tuned in on the 2nd radio but admittedly I may be a little relaxed due to being on Flight Following and knowing that the local NORADs dont generally come up to 9500. I start a slow decent 32S, Im at ~8500 now then out from under my wing pops a Cessna, 1/8th of a mile away going direct to 00W. I had a momentary flip out and decended below him and he must have gone right over the top of me, couldn't have been more than 100 feet probably closer to 50. I got a little excited on the radio to spokane approach because I lost visual on him and had no idea what his intentions were. Spokane approach couldn't see him. I recognized the plane and alerted the flight school that it belonged to that their transponder may be INOP as we were directly over the MSO airport and FF had no problem seeing me. I'll be looking a lot closer. I never heard a peep out of him, CTAF, tower didn't know they were there nor did Approach. I know, I know see and avoid.