Flywheel balance? I'm short on ideas at the moment.

I get a hum similar to what you've described under similar circumstances. It's been there since the manual transmission went in. I've never done any troubleshooting on it, since mine isn't very loud and I kind of like it. I've long suspected it has something to do with the fact that I'm running a 616 flywheel on a 617.

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 401,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 26,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.
99 Mazda Miata 183,xxx miles.