This is going to sound cray, but I am wondering if it is coming from the fuel return line going to the tank. Does it have the cigar hose or has someone replaced the return line with a piece of regular fuel hose? It will resonate like hell and gives the characteristics you describe. Mine did it and drove me nuts until I replaced it.

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Thanks,
Mark in NC

"Spark plugs?...We don't need no stinking spark plugs!"
1985 300SD "Der Silberne Schlitten" 420,000 mi


Wish these were diesel:
2003 Ford Club Wagon 130,000 mi