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Motor mounts? Other than that, I concur. The original B18 in my PV started making a sound exactly as you describe - sounded like an old diesel engine. It would even fade away somewhat if the revs went up. This was near the tail end of a highway trip, and I pressed on another 30 miles or so before getting home. And then I drove it about another 15 miles back and forth to a shop (who very kindly diagnosed it as a timing gear within about 10 seconds of listening, but was too booked to do the job).
When I took the timing ocver off the fiber portion of the gear was so loose on the hub that it practically fell off. The only thing keeping it working was that the steel hub had some fairly large splines that were still engaging in some grooves int he gear, and the torque of the gears against the tooth angle tended to pull the gear back against the hub. But it was definately on the N-th stage of failure, I would have been lucky to get 10 more miles out of it, best guess, before the abused remnants of the fiber gear decided to not pplay with the hub anymore. I think the most abusive time for the gear, oddly enough, was at idle. When going slow the oil pump has less drag and the valve springs can push the cam forward as they pass 'TDC' on the cam lobes, then the cam yanks backwards as the next lobes starts pushing. This back and forth really exacerbates any slack on the cam gear or the thrust block. When revved up, though, the oil pump makes more drag (my theory) and the cam gear gets into a constant pull - thus the lessened clatter.
My advice - if you don't fix it now at home or you'll be paying for a tow from the side of the road. Luckily enough if the gear totally goes it won't damage the top end of the engine - the pistons don't hit the valves of a B18/B20 that is anywhere remotely close to stock. (Shaved heads, decked blocks, big valve lifts - GOOD LUCK!)
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I'm JohnMc, and I approved this message.
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