Tunnel slap!

[Mike Grace <mgrace@domain.elided>]

I've got a scary-sounding problem with my '74 GTV. When I hit nasty
expansion joints in the freeway at 70 mpg (indicated), I frequently get a
severe WHACK! from the driveline slapping into the side of the tunnel. It
seems to be the donut joint which hits, but the U-joints, donut,
trans-mount, and motor mounts are all new, the driveline was recently
dynamically balanced *on the car*, and there's zero (or minimal) vibration
felt at speed. The center support is not new, but it appeared good to me
and my mechanic. What's up? This dynamic interference is difficult to
pinpoint, but definitely unacceptable.

Thanks for any help-
Mike Grace
Santa Barbara

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