Keep after them, tell them you will not accept an end to warranty until this problem is fixed. Also ask how many calls it takes before they replace the unit.
Often they have senior techs request one of these make the next call.
Since they have already replaced the drum, they have acknowledged that there is a problem so cannot use the "This is normal" excuse
The rumble means something is moving in a way that it should not and it is very likely this is causing wear on other parts.
Could be caused by a number of things.
1. A support roller with a flat or soft spot
2. The front supports/glides
3. A roller shaft that is not 90 degrees to the frame. The rollers run in a channel, if not at 90 the roller can ride up the channel, gets so far then will fall back into the center of the channel
4. Anything to do with the drum that is out of alignment
And probably more that I have not thought of
Put the unit into air only
Open the door and hold the door switch closed
Run the unit
You may be able to see what is happening to the drum to cause the rumble
re: side to side movement, bouncing at the back etc.
Relay this info onto Whirlpool for the next service call | 
