Scrambler

I'm new to MBWorld (today) ... but I'm trying to determine the origin of some annoying wheel noise. I purchased a 2 year old ML350 4-matic for my wife a couple years ago with 20K miles and have recently observed a howling wheel noise (as if we were driving a Jeep with huge knobby tires) I don't really drive her car ... but we took it on vacation recently and I was shocked. Here are a few facts:

• She noticed the noise about 6 -8 months ago and took the car to the MB dealer at least twice. They say it was the tires.
• The tires are fairly new Michelin Latitude tires that I purchased NEW at ~ 35K miles. They are balanced, regularly rotated and were rebalanced / rotated again after our recent vacation.
• Now ... I'm headed to the dealer for a more 'in-depth' analysis. I really like this vehicle but need to fix this issue. I can't help but think this may be somehow related to the 4Matic drive and they are procrastinating some potential warranty work. I hope that's not the case.
• The vehicle has ~ 53K miles now and the warranty is for 100K

I'm posting this b/c I'd like to understand any outlying issues that I may not have covered. Frankly, if the dealer is still adamant that it's 'tire noise' ... I'll have THEM put 4 new tires on and then CHALLENGE THEM to quiet the car. Any thoughts are greatly appreciated