Hi,
How many miles is your car? A groan while driving? IMHO it could be some rubber suspension parts. I think a low rumble is much more likely to suggest front bearing wear. My 1994 940 still uses its original front bearings now at about 860000 km.
The replacement front bearing comes as a sealed unit. I don't think any re-greasing could be done for this bearing type. That being said I've replaced my rear bearings with Optimal brand from Germany at about 800000 km many years ago. I replaced them after hearing slight rumble at the back. But the rear OE ones still have some life in them (only slight visible wear) and could have done some re-greasing if I wanted to. As the half-shafts were already out I replaced both of them anyway.
There are also Optimal front bearings for the 900 series. I think they are good enough if you don't want to go OE. Anyways OE is always better. Just my experience.
Regards,
Amarin.
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