I have been using my home made vertical tube suppressors ..previously pictured above. They Work: HOWEVER: They need to be drained every three months or so..depending on the frequency of use of the washing machine. Shut off the water main and draining takes about 15 minutes.
I'm now looking at commercially built
"Mini Trols". They have a rubber diaphragm that separates the water from the air.
I haven't done the change out yet because the "Mini Trols" are about 1/16 inch TOO big in diameter to fit where the old vertical tubes are. Need to do un soldering and then re soldering of the cross connectors and elbows.
Soldering wet pipes is A PITA.

NB
Mini Trol suppressors:
http://www.amazon.com/Amtrol-Mini-Tr...mer+supressors
I have found them locally for less than $50.