[Richard]

I have a that noise upon start up in one unit. It lasts about 2 seconds and goes away. I have had the unit part twice with no luck in pinpointing the noise. No signs of either squirrel cage hitting. I am only guessing now that one of the many sheet metal pieces is resonating as th blowers come on.

While you have the unit out measure the amperage on both compressors and both blowers. Should be between 10 and 14 on the comps and under two on the blowers. Anything higher than that may require further investigation