For several months now, my 2002 outback has been randomly generating a deep rumbling noise and accompanying vibration. The best I can describe it is similar to crossing ripple strips, but if they were shallow. If I maintain speed, It will slowly increase in frequency for a minute, then decrease and disappear entirely. If I decelerate, the rumbling also decreases, as if I’m hitting fewer and fewer bumps. If I come to a stop, it disappears entirely, and doesn’t reoccur when I accelerate afterwards.
When it occurs, it seems as if the entire car is vibrating/shaking. I can definitely feel it through the steering wheel, and sometimes a slight squeak will occur from the front passenger side (due to the randomness of the event, it’s difficult to ascertain whether or not there is always a squeak).
By random, the symptom would appear generally once a month. The longest span without it occurring has been at least three months. This has happened at speeds of 50, 60, 65, and 70 mph. I’ve been told by friends that it might be a wheel bearing, and a mechanic has said that my heat shield was loose (and there was nothing they could do or needed to do about it). But as random as it is, I’m uncertain what the cause is.