[Richard]

You will have to bleed all the air pressure from the coach to do this. Take the coil off by unscrewing the thumbwheel on the right side in your pic. Then take the valve apart, being careful to keep the parts in sequence. Clean up the parts, and lube them very lightly with silicone. Reassemble and see if that fixes it.

I think that this is the valve that exhausts air after the compressor cycles. Right? So you are blowing wet air sometimes, and any trash from the air system through the valve. I am guessing it's gunked up on the inside.

If you by a new one, it will not come with fittings, so make sure if you want to use the exact setup you have to buy one with the same size pipe.