Hmm, I think I have the same problem. I described it in my Bosio feedback thread, but I didn't mention that in addition to the highway pinging I also sometimes have surging at the same time. I'm starting to suspect the flaps. As I said my EGR flap doesn't move anymore. The other flaps are harder to check for proper operation since you can't see them but I did lube them recently.

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2004 VW Jetta TDI (manual)

Past MB's: '96 E300D, '83 240D, '82 300D, '87 300D, '87 420SEL