A thumping heard and felt in the floor is often caused by failing engine mounts. A failing mount allows too much movement of the engine, and will require replacement.
Problems for specific Nissan Quest years:
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Newest reported 1998 Nissan Quest problems
A coolant leak may develop from a plug on the front of the rear cylinder head, behind the timing cover.
The distributor shaft bearing can become rusty, and seize causing the engine to stall or not start. The failed distributor will commonly require replacement.
The exhaust manifold often warps and cracks, breaking off the studs between the cylinder head and the exhaust manifold. The noise may be most noticeable when the engine is cold.