When you mention rumble, do you mean a vibration type of feeling/sound, or a rotational WUH WUH WUH type of sound.
If it sounds like an out of round tire, it may just be that the tires need to be high speed balanced or balanced on the car.
Try rotating the tires front-back and see if the noise is still there.
Otherwise you might be looking at a bushing that may be dry and cracked or worn, or the possibility of a bad shock perhaps.
I would have the suspension checked, balance chacked, and 4 wheel alignment checked.
In my case, I have a sound like an out of round tire, but my wheels have excessive toe in from a visual apperance. mine is more of a rotational noise, like an unbalanced tire, only my tires are in balance, and I get my noise at any speed.
Mine may be differential noise, but to me its more likely crappy tires and out of alignment. if afte rI fix that stuff the noise is still there, I'll work on differential.
Perhaps yours is a problem with the driveshaft being out of balance or a bad flex disc.
If the car sat a lot, have the rubber in the suspension / drive shaft checked.
'92 300CE - Sold
2004 C240 - C7 Wheels
2015 ML350 - P1, Pano, Ash poplar wood, Sport Dinamica interior, Running Boards Keyless go.