I have a Lennox variable speed air handler that was installed 1 year ago. Occasionally when the unit kicks on the fan does not start immediately. Instead I get anywhere from 3 to 30 seconds of the handler making a loud thumping noise. Almost as if the fan motor isn't starting up correctly? It will thump about 5 times then briefly pause. Then thump 5 more times then pause. It will continue to do this over and over until the blower kicks on. At that point everything sounds and behaves normally.
I had the AC company come out and inspect it and they state that everything is just fine. All of my various pressures and voltages were A-OK. I explained the noise to them (naturally it didn't do it the 2 times they turned it on) and they said it was perfectly normal. Eventually I will get used to the noise and won't even notice it.
Variable speed motors will rock/thump on startup and that is the nature of the motor and normal. It is charging capacitors internally and determining an initial speed setting and getting ready to ramp up in speed. Part of the PWM Pulse Width Modulation process.
"Cut it twice and it is still too short".