rattling under MG Zt

I noticed a rattling yesterday coming from under my car. It seemed to be somewhere on the driver side, but that could just be the acoustics. It speeds up/slows down in sequence with acceleration. I've had a look underneath, mainly to see if anything was simply caught on one of the wheels, (that's what it sounds like), but there's nothing apparent. Just now, after being out for an hour, it seems to be possibly louder, not sure, and also to have moved to a point more centrally under the car. Also the noise seems to be changing from a tinny rattle to more of a dullish sound.

Getting a bit concerned!! Any ideas???

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