Rear Suspension

I'm on my second NC.  The current one is a Sportstech RC the previous a Sport (for three years).  Before that I  had an 1800 Auto for four years.  So I've been round the block a few times in an MX5!  I have neglected the forum for some time but it is interesting to see that the dipstick readability problem is still there.  I raised the subject about three years ago.

Anyway my question "Has anybody noticed what I can only best describe as a 'trundling' or 'rumbling' noise from the rear suspension/wheels when driving over a particularly rough surface."

I had the problem first with my NC Sport which the dealer mechanic coud hear but without being able to find any problem with the car.  When I picked up my current RC Sportstech six months ago it was heaven with no worrying noise! However, over the last few weeks the noise has started to occur on this brand new model.  Does anybody have any ideas?