I have a 7 year old carrier central airconditioner.

In march, 2012 all Freon tubing and copper lines were replaced. In dec. 2012
A motor module was replaced. Since these events, we hear a "mini" explosion
In the air handler once a day and only during air conditioning season.

Could there be a bubble in the Freon lines?

Any help would be appreciated. My service company is stymied.

Is the source of this sound coming for the outdoor unit or the indoor?

Indoor

Are your ducts metal & do you hear this noise either when the fan first comes on or just after it shuts off?

Sounds like "oil canning " of the duct. Have someone listen along the duct as the unit cycles. You can usually correct by hitting the duct with a rubber mallet in the area where the noise occurs. This rebends the duct.

The noise is coming from the air handler, which is in the garage. It occurs once a day, a loud slamming noise, usually towards the end of the day.

Sounds like "oil canning " of the duct. Have someone listen along the duct as the unit cycles. You can usually correct by hitting the duct with a rubber mallet in the area where the noise occurs. This rebends the duct.

It's definitely not coming from the ducts.

Noises can be difficult to find & fix in person. Trying to fix one online is nearly impossible especially if it only happens once a day

Is the unit running or just turning on or off? Still sounds like ductwork, perhaps in the return. Filter clean? Dirty filter causes negative pressure in the air handler. There is nothing in the air handler except a filter and ductwork. Check the filter mounting.