My wifes A170 has developed a pronounced rattle which seems to be under the drivers seat ( RHD ). It sounds as if something is bouncing around on top of the platic undertrays, but when I take them off - nothing !
Any ideas . folks?

Greetings SunBurst,
Did you mean that the rattle noise stopped after you removed the plastic undertrays or nothing happened (still same problem?)
The trays are a little bit tricky to put back and be sure that you have done it right. There are 2 slides on each side and be sure you have attached them correctly.
Sorry for my poor english!
Regards,
Lummi

There are two plastic brackets that help support the side skirts, they will break if the car is lifted by the sideskirts. take the covers off and look there.