I had the same thought upon reading your post. If it sounds like bb's in a
plastic bottle, it's engine detonation. Older V-8 in luxury cars are especially
prone to this as the carbon build-up on the pistons reaches a threshold from low
RPM only operation. There are professional engine decarbonizers available. The
other solution is to run premium fuel, and make sure to floor it more often.
That sounds like pinging, which could be caused by a number of things,
including a timing problem, cheap low octane gas, or an overly heavy foot.
Except that the Caddy 4.1/4.5/4.9 never *HAD* springs in the
distributor. The computer controls timing on those.
They are notorious for pounding like someone's sledge-hammering the
pistons if the EGR valve sticks closed. Thats where I'd start.
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