Grinding in 4th & 5th ???

[T. 12 E E D.]

I was on the motorway tonight and it was pelting down with rain and when letting off the accelerator in 4th and 5th there was a terrible grinding which sounded like it was comming from under the car on my way back on the motorway the rain stopped and the grinding also stopped has anyone experienced this in there 182 or can anyone shed some light on the problem ???

Thank's