Dear Tom and Ray:

I'm from Huntsville, Ala., the home of the Space and Rocket Center and
Werner von Braun, the famous German rocket scientist (knowing this tidbit
now qualifies you as an historian of Huntsville!). Anyway, I have a 1991
Mazda 2600i pickup with 63,000 miles. Recently, it was brutally cold here,
maybe 6 degrees F. I left the vehicle parked outside, not in my garage like
I usually do. The next day, I started the truck and drove it to work. I
noticed that at speeds over 40 mph, there was a sound I had never heard
before. The sound it made was the same sound you hear when you drive over
one of those grated steel bridges -- a rather loud humming noise. The sound
is only noticeable at speeds over 40 mph. There is no difference in how the
truck starts or drives, except for that awful humming noise. Did something
happen to it the night I left it out in the cold? -- Kim

RAY: First of all, Kim, my only knowledge of the famous rocket scientist
Werner von Braun comes from satirist Tom Lehrer, who wrote this
tongue-in-cheek musical tribute to Dr. von Braun, and the way his
groundbreaking (literally) research was ultimately used by Germany in World
War II:
"Some say our attitude,
Should be one of gratitude,
Like the widows and orphans
in old London Town,
Who owe their large pensions
to Werner von Braun."

TOM: Werner von Braun aside, Kim, the problem with your truck probably
didn't happen on any single night, but it may be related to your recent
cold spell.

RAY: It could be a number of things, but I'll give you two possibilities.
If the grease in the axle bearings got congealed in the cold weather and
wasn't "loose" enough to do its job, one of the balls in the ball bearings
could have gotten spalled (roughed up in one spot). Once a ball gets
spalled, it can cause the bearing to make a humming or growling noise from
then on, and that noise is often most noticeable and most constant at
higher speeds.

TOM: So have your mechanic take out the axles and spin the bearings to see
if one feels rough. If it does, rejoice, Kim. That's a relatively
inexpensive thing to fix. An axle bearing can be replaced for about $100.

RAY: If it's not an axle bearing, something may have happened inside your
differential, which is much, much more expensive to fix. So if I were you,
I'd definitely go to my favorite house of worship tomorrow and light a
candle for an axle bearing. Good luck, Kim.