If you're hearing a growl or buzzing noise at 40 mph and above, and the tone varies with vehicle speed, not engine speed, then it's probably coming from a rear wheel bearing. The bearings themselves are inexpensive; however, they have no inner race. Instead, the rollers ride directly on a machined portion of the axle, so if the surface of the axle is damaged, you'll need to replace it. (Yes, I'm aware of the availability of "repair bearings" that are offset to ride on a different part of the axle shaft. I have no idea if they're any good or not. I prefer to repair to OEM conditions.)The trick is that you just can't know whether you need a new axle (or two) until you get them out and inspect them. Fortunately, next-day shipping with AutoPartsWAY Prime is available at reasonable cost, so you can disassemble on day 1 and reassemble on day 2.My Grand Marquis started making the buzzing noise at about 150,000 miles, and it was caused by galling on both the...