I thought the first time was a fluke but it happened again today right when the rain started, the same as before.

I'm driving the car and everything is going fine but when it started to drizzle, very loud dead thud sounds started coming from the rear as though I had 2 flat tires. Every Small crack in the road resonated through the car. I also had to steer it with both hands because it felt as though it could jolt left or right at any minute.

What could this be?

This has only happened twice and both times were on rainy days and both times it began as soon as the rain started.

I'm sure I need the rear subframe bushings but I don't think that would make this kind of issue only in the rain.

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2000 X5 4.4i w/ 150K miles.