I live in a three level townhome, in N. Virginia; 2.5 ton A/C condensing unit outside and a forced air furnace in basement inside. HVAC specialist changed the blower motor to the inside unit and added freon to the outside unit. The A/C produces cold air. The heater produces warm/hot air. But, after he left, the furnace started making very loud noise, regardless whether it's on cool or heat. Please comment.

Could be the inside blower motor. That will have to be checked. Call the company back and tell them the unit is now making a noise inside. Since they were just there they might cut you some slack on the new bill for the inside repair.

Service people do not have a crystal ball so these type of things do happen.

One thing you can look for is at the blower assembly in the furnace. Sometimes a piece of paper(usually the furnace diagram will come off the blower door. That can make a noise like you are talking about.

Summer is right around the corner so get it fixed now before the rush.

Thank you Mr. Hvac. Are you saying the inside unit, the furnace, has two blowers?

NO
It is possible that there are two cage blower wheels in the indoor furnace but only one motor. If it has two cage blower wheels check both sides and BE SURE to turn off the power to the furnace before messing with it.

If he changed the blower motor on the inside and it's making noise, then call him back. He could have very easily not tightened something.

About the two motors, there are lots of possible answers from:

Two circulate the air
1 motor and two blower wheels
1 motor and 1 wheel

If a high efficiency furnace then there is an draft inducer motor

And on the outside
There is a condenser fan
And a compressor motor.