Yutaka,

Definitely sounds like air problem.  Got 1 Hz or less (usually 0.5 Hz) when had air problem.  After running lateral a bit it would disappear, then if actuator sat still w/ high pressure for about 5-10 minutes then could hear the low frequency come back.  I agree, 80 gal is way too much.  The pump should be checked for foaming; the mods may have not done any good.

At 09:43 AM 4/10/98 -0500, you wrote:
>Hello Rod-san,
>
>How are you doing? After modify the HPS, the Y2 actuator(closest to Z3)
>squeaky sound has occured(move the actuator round 1Hz),after bring up
>the High pressure.
>But it has gone,when we did random. (But it happens all the time now) So
>I checked both 469D program. These were nothing wrong. I changed Valve
>driver, but it was OK.
>I wanted to change the ServoValve,but we don't have any spare for that.
>Custmer still don't know it happen all the time. 
>I really wondered, we modifyed the HPS. It seems getting worth. (It's
>about 80gal maximum) Please help!!
>
>Best RGDS,
>Yutaka 
>