I was getting a strange vibration/rumble while coasting down from about 40, which was also noticable at about 75. Turned out I over-torqued my dog bone mount, which was sending too much vibration into the cab once it had settled down/worn in. It sounded just like a bearing, which a local garage replaced, but to no avail.
Its something quick and easy to check, and while its off, you may as well do it for the sake of £30 if its not been done already.
Hope this helps,
Dan