(help) metallic rattling/grinding sound when driving in the rain

[Dylan_K]

hi guys..not too sure if this is a common problem, but would like to seek some advise/help.

I recently noticed a metallic rattling/grinding sound when i was driving in the rain (it was raining almost every minute for the past weekend over the new year), which was never there when I was driving (slow/fast, easy/hard). This was on the expressway where there was quite a bit of water puddles/ponding. it was loud and i can feel the vibration from my seat, and it felt like it came from the undercarriage. so obviously I stopped the car, got out and looked underneath but there was nothing dangling or loose. When i drove off, the sound was still loud initially, but after some gentle low/rpm cruising the sound disappeared. This sound was not there when i drove previously, and is not there now when i'm driving. no engine light came up, and Xentry check turned up nothing too. 

i saw a few bros in the group who shared that their undercarriage suffered some minor damage from driving thru puddles, but i don't think mine's the same case since i don't see any thing loose from underneath. no obvious fluid/oil leakage in the carpark also. i googled and some other folks shared something about water causing the exhaust to contract etc, which sounded abit strange to me, hence asking here to see if any one else had a similar experience and had more success in troubleshooting. 

TIA!

[Timhochee]

Bro did u manage to find the root of the issue? Experiencing same on my W212