after a few years, yanmar mounts can be adjusted using the threaded rod. the adjustment of the roads takes up the slack created when the compressible rubber gets smashed down by repeat engine vibrations. when you can't adjust the threaded rods any more , it's time to replace the mounts. i got 8 years out of my 2GM20f engine mounts,(500 hrs on the engine) . at that age they were ' 'bottomed out and not adjustable using the threads any more, then there was quite a noticeable increase in engine noise
and vibration. if oil
dripped on the mount directly under the oil
filter you could have one bad mount as the oil degrades the rubber..but at those ages it may be time to replace them all. not a bad job, just a pain. mounts for front (beltside) are different stiffness than for mounts at rear (gearbox side) so buy the right ones and keep them straight while you install them.
fair winds to you.