I have a Trane A/C that is 7 yrs old. Live in North Texas, so it gets plenty of use. 18 months ago I had the blower wheel replaced because the "spot welds came loose." Today, I was having the unit checked out after a lightening strike, and he said, "your blower wheel is a bit noisy - looks like the spot welds have come loose." I mentioned that his company just replaced one, but he said it's out of warranty now.

Am I having a run of bad luck here or is this a common problem?

Sounds like a run of bad luck. I don't recall ever having to change because of bad welds

Have your contractor check the units total static pressure, if it is too high compared to the units blower spec's, it will lead to early blower wheel and / or motor failures... as I have seen on a rare occasion, on furnaces and a few comm. Rooftop heat / cool units. ( NOTE: wrong duct sizes for proper air flow is usually the root cause for this type of failure. )

Thanks - I'll have them check it out!

Your HVAC service provider should stand behind the original installation and the replacement work already done. Contact Trane's CS, if necessary.