It could be preignition or maybe even a clogged injector causing a cylinder to lean out and detonate. Try running two quarts of Chevron Techron Concentrate with a full tank of gas. You may have to do it a few times. Make sure you're using 91 octane too, maybe from a different gas station (to rule out bad gas).

When you hear the "slap" you should back off because, if it is preignition or detonation, it's definitely not good for the engine. After running that much Techron, you should consider changing the oil. I don't know if it's true but some people claim that the Techron adversely affects the properties of the motor oil and others say that the deposits get into the oil.

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles