[AlCan]

Good point, AlanM! Many thanks.

I took a look at the mount, which seems to be okay. It has about 8mm or so of (vertical) clearance between the bottom of the "tank" and the mounting bracket below it, and no sign of leaks so I think the mount itself is ok.

Then I remembered that when I changed the engine oil, discovered that the lower / rear mount had scrape marks from taking a good hit at some stage. Tarseal was jammed into the bracket and between the mount and the cross member. Cleaned it all out at the time but didn't think too much more about it.

Maybe that hit has pushed the cross member back a bit and put all the mountings out of alignment. Took one of the chassis bolts out of the top engine mount bracket to see where the mount was sitting relative to the bolt. Almost as far back as it would go. Undid the other and pushed the bracket right back and retightened.

It has changed and improved the problem. I'd say that cross member must be bent, as now, there is generally much less vibration until you put your foot down harder. Then, guess the engine moves more and the hole in the bottom of the tank connects with the bottom post - and voila, much vibration. Back off a bit and it goes away.

Another thing to sort out. Thanks again for the tip.