My Forester Snorts

I have a 2000 Forester L, A/C, manual, clock reads 138,000, but my mechanic claims the engine is significantly newer than that. Almost like a misfire, or a backfire through the carb (which it doesn’t have), across my four-and-a-half mile commute to work, my car will go ‘PFF.’ Sometimes this eructation is loud enough to be heard over Dianne Rheme, sometimes it is barely audible. It does not appear to be related to engine rpm, actual speed, or even if the car is moving, as it will snort at idle. On cool, pleasant days it will snort less, on hot summer days under load (with the A/C) or in the grip of winter, it will snort more.



This snort/stumble does not seem to correlate with a dead miss, but it is annoying and makes me wonder if some component isn’t about to fail. My local moro… I mean mechanic charged me $400 in diagnostics and shrugged his shoulders. A nearby Subaru dealer charged me $360, changed my knock sensor, said that there were no codes and it was fixed. Then it snorted as it sat there running next to us. He shrugged his shoulders and said, ‘try new plug wires.’ I almost spit on him, I said, ‘you’re a Subaru dealer,’ he said, ‘uh, yeah?’ ‘and you don’t know what’s wrong with my car?’



I did change the plug wires, and it made no difference. The oil, filters (fuel and oil) and plugs are all new, factory recommended parts.



Help?

Terribly sorry, sentence two should end, "…my car will go ‘PFF’ nine or more times.