The thumping noise under the dash is caused by the blower motor probably hitting the basket that goes around it. The basket may have debree in it an the fan is hitting it making that thumping sound. An the blower motor itself may be going bad. It should be located underneath the glove box check it out an let us know! Good Luck.
You could also be hearing the expansion valve pop just as the pressures rise to operational range, look at where the valve is mounted and see if it is too close to the firewall without any rubber buffer material between the valve and the wall. Do you know if there has been any heater or A/C work done?
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