Coolant

My mechanic changed my thermostat recently. I am getting an unusual sound when the truck is moving forward coming from the bottom. This only occurs when I accelerate. My mechanic and a different guy hear the sound but cannot identify the source. This morning while the engine was cold, I noticed the reservoir and were both filled to the top. Is this normal? A good friend says there is air in the system causing the noise because they are both overfilled.