Heat Pumps and Electric Heating - Loud hum in lines on heat

We have a Coleman 3 ton heat pump with an indoor air handler. It is 13 years old. Recently the unit quit working and the red light stayed on. The heat and air people found that the defrost control box was bad. They replaced the control. When they checked the pressure in the lines, they found them to be restricted. They vacumed the lines, replaced both P-Stems, the dryer unit, and refilled the freon. The air condition mode is fine, but when you switch to heat you get a loud high pitched hum in th line(s). They had never experienced this problem before, so they sent their best man out to check it out. He checked out their work, taking the fittings loose and all. He is also at a loss and plans as a last resort to try and cut down the pressure in the lines. Are there any suggestions out there for this problem?

When they checked the pressure in the lines, they found them to be restricted. They vacumed the lines, replaced both P-Stems, the dryer unit, and refilled the freon.

From that Id say. Get a different company to work on your unit for sure. I didnt put gauges on the unit. But I cant see that a bad defrostboard would cause all that. 13 years old is about at its end. But a lot of them run longer.

Thanks for the reply. There has to be something we can do to stop that noise. They say the pressures are correct. Is it possible that because it is 13 years old, and now with no restrictions, the pressure in the lines is causing this problem?

He checked out their work, taking the fittings loose and all. He is also at a loss and plans as a last resort to try and cut down the pressure in the lines
I just dont get what all they are doing here. Why or what fittings did he take loose??? Did they move the unit a little or the lines. Is 4 way valve ok or on some units it has to work on heat, then it might hum. Id check it.

The fittings I'm talking about are the ones that house the Pistons. He said that they always replace these (the pistons)when there is a restriction in the lines. They did not move the unit or the lines. I don't believe they checked the 4-way valve. He is coming back, we'll see.