Is it possible that the old engine mounts/shocks were worn to the point that they more freely let the engine move w/o moving the rest of the car at idle? Now with new firm mounts, the vibration is transferred to the rest of the car...more?
Just a guess.

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Chris
1985 300SD - 'Grace' (198K mi.)
2018 Honda Civic Sport
2018 Honda CRV LX
2010 Honda Fit Sport (RIP)
2013 Honda Accord Sport (Sold)
1996 Lexus LS400 (Retired)
1995 Ford Contour SE (Retired)
1976 Porsche 914 (Sold)
1972 Datsun 240Z (RIP)