Jamie,
The mounts are filled with ethelene glycol (antifreeze) and they leak. You can measure the mount to see if it needs rplacing. The bottom edge of the metal on the mount should be no closer than 12 mm from the subframe. If you had clogged cats, you should have experienced a power loss. You can check for exhaust restrictions with a vacuum gauge. Hook it up to intake manifold vacuum and run the engine up to about 2000. The vacuum should drop off and immediately recover to above idle readings. If it is lower than idle reading, you have restricted exhaust or late valve timing.

Peter

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