[moltuae]

I had a similar issue some years ago. IIRC, in my case it started just after I'd had a set of new tyres fitted (on newly refurbished wheels). There was an intermittent vibration, which seemed to come from the rear. It would usually happen after hard acceleration, typically just after joining a motorway. The vibration was severe enough that I pulled over onto the hard shoulder the first couple of times it happened, expecting to see a wheel falling off or a bulge in one of the tyres, but I could see nothing wrong. I gingerly proceeded and the vibration was gone, until the next time I stomped on the accelerator.

I wondered if one of the rear wheels might be slightly bent so I spoke to the wheel refurbishers about it. They told me that they check all the wheels 'run true' before refurbishing so this was unlikely and suggested that I have the alignment checked. So I had my usual garage check the car over and perform a 4-wheel laser alignment, which seemed to make a big difference. They also noted though that some of the rear subframe bushes were pretty knackered. Several months later I had the rear subframe replaced and all the bushes renewed. I'm not sure if the cause was worn bushes, alignment or a combination of both, but the problem has never returned and, following the subframe replacement, the car now feels much more 'planted' when corning.