Since I often search here first and I was unable to find a posting, I wanted to mention this to anyone else with this issue as it is really easy to fix, but really frustrating while it is happening.

Even if you have the older "Water Hammer Arresters" (a built-in vertical stub of pipe) near the washer water supply pipes.

Sometimes the air inside gets "lost" (absorbed into the water).

You'll need to turn off the house main water supply.
Then open an upstairs H&C water faucets, and then the hoses by the washer, to drain the water out of the pipes (and thereby, the arresters).

Then close everything back up, and turn on the house main water supply.

You'll need to purge the air out of most of the household faucets by letting them run for a few minutes.

The older style Water Hammer Arrestors don't work very well (no rubber bladder).

May need to be purged every few months..

New style
Lowe's $ 10
you'll need one for each Water Hose ...
click on picture

So far we've tried a water hammer arrester, changing out the hose from the wall to machine for shorter (4-foot) hoses, and now we're trying a garden hose from the flush outlet on the hot water heater directly to the washer. (Following the guidance at http://lgknowledgebase.com/kb/index.php?View=entry&EntryID=4415.)

Any suggestions would be greatly appreciated.

One on each Hot & Cold water supply ?

The closer to the Washing machine, the better.

The ones pictured above connect directly to the Water Input Vales of the Washer.

Works great on my LG WM2688