I wouldn't think the gas sloshing around would be very
noticeable...aren't there baffles in the tank to keep that from happening?
A more likely (IMO) explanation might be the car shifting when
accelerating. Firmer shifts are more noticeable than softer ones.
Also, there might be a loose connection somewhere under the car...a
U-joint that is not properly secured...and whatever is being held by the
U-joint bounces around under the stress of acceleration.
My $.02 worth.
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