Well, the noise doesn't sound right, but the location and occurance do: when
I replaced a motor mount, my 'B developed a knocking sound on the bottom
left under identical de-Reving conditions. Turned out to be the exhaust
hitting against the cross member. The clearance was such that it would only
make contact when the engine would de-torque but still be vibrating fairly
heavily.
When I checked the exhaust clearance, I "pushed" the pipe to see if it would
make contact, and it didn't. The mechanic I consulted banged it with the
bottom of his fist and reproduced the noise no-problem. The fix involved a
large screwdriver and less expense than an oil pan gasket.
Could be a broken bolt on the manifold/pipe join as well.
CJC
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