It is annoying. I've had this problem w/ my '93 850 since I bought it in 1995. It comes and goes. Won't hear anything for weeks and then it'll start on a trip and "honk" -- a real metallic groaning -- at irregular intervals until I get tired of it and, weather permitting, turn it off. Mine will occur with the AC compressor either on or off. But if I shut off the ventilation system completely, then of course it goes away.
Based on Len's experience, I think I'll just continue to live with it. Sorry I can't offer any other solution.