My 2002 Golf TDI, 80,000 miles, last year started having exhaust coming in the cabin if windows were open, and a bit even when windows closed. Had mechanic look it over several times, but they cannot find problem. Intake manifold was almost completely clogged-- had it cleaned out-- anti shutter valve has stuck twice-- leaving me stranded. Changed all filters. Why is it getting so gunked up, and where is this excessive exhaust coming from??