SW,

The pump impeller is probably cavitating as you
suspected, which means that the flow is restricted
enough from going from two inch piping to 1 1/2
piping and so is pulling a vacuum on the water and
beating the subsequent bubble that is created
making the noise. This is pretty much confirmed
since the sound goes away when you shut down the
discharge of the pump decreasing the flow
requiring the pump to push less through the lines
meeting the flow requirement for the 1 1/2 inch
lines.

In this instance you have two choices, either
replace the piping to the pump increasing the
diameter to 2 inches for all of your piping or buy
a new pump with less discharge capacity. I guess
there is a third choice and that is leave the
discharge shut down to the point where the noise
stops.

The thing that you have to remember, if the
cavitation is allowed to continue (I do not know
what kind of pump you have) there is a chance that
the cavitation will damage you impeller requiring
pump replacement or impeller replacement.

HTH

Tom L.L.
=============================
"SW" wrote in
message
news
I have an inground pool. 2 inch plumbing
throughout, except necked
down to 1 1/2 right before the intake to the
pump. The output goes
through a filter system which has pressure
gauges and then to the pool
return line (2 inch) and the spa return (also 2
inch). The pump is
putting out 1 1/2 to 2 hp. When the pump runs,
it sounds as if there
are bubbles in the intake line. NOT the pump
itself, but the intake
line leading to it. I don't see any air in the
pump or lines. When I
close off 1/2 of thr output by closing down the
pool return, leaving
only the spa line open, the presssure in the
filter line jumps from 15
psi to 22 psi and the noise in the intake
instantly stops. Open the
pool return line again, and the noise instantly
returns. Is this
cavitation? The motor seems to run hot, but has
had no problem putting
out flow or tripping breakers. What can I do? I
can't reasonably
change the intake to 2 inch without spending big
$$ as all the valves
will need to be changed.

Is this cavitation or an air leak. I've never
seen cavitation before ,
and know little, but it sure seems like it to
me.