A3 8P ('08 sportback) strange thud noise 🤔

Thought I'd post here for ideas, as it's a bit of an unusual one. I've been borrowing my mum's '08 A3 sportback (2.0TDi 140 DSG Sport) for the past few months/few thousand miles. 145k miles, runs great, I always look after it.

The past few drives, I've noticed a "thud", from the front wheel-well area. The first time, I thought it was just going over debris on the road that I hadn't noticed, being thrown up and hitting the well or undercarriage. Then it happened again, maybe 30 mins later.

I forgot about it, but then again today, twice in about 90 mins of highway driving, this brief, mid-pitched thud, as if something is hitting the wheel-well or undercarriage. It may be exclusively from the passenger side, but I'm not sure. Car seems to drive fine otherwise, no issues or noises cornering, the thuds were all when driving straight.

Any ideas what might be causing this? 🤔