Calling Netmax... Fluval 404 noise problem

Netmax,

I know you almost swear by these filters... I have one that is about 3
months old... keeps sucking air in somewhere... cycles about a minute
or so apart... gets noisey, then spits out a bunch of air bubbles...
if I rock the filter I can hear them being spit back out again and
after a few minutes of rocking it back and forth the noise stops for a
little while anyway... then the whole process starts again.

I have used silcone lubricant on the only "O"ring that came with the
filter but I actually think the air is coming in from elsewhere. More
than I can account for by ingestion from the intake. I do have lots
of very very tiny bubbles generated by a bio-wheel Hot Magnum Pro
filter on the same tank but these bubbles are very tiny and I seldom
see them actually making the trip to the far side of the aquarium
where the intake for the 404 is.

IE: intake for 404 is in far left back corner of 4 ft tank, bio-wheel
is about 3 horizontal feet away and 18 inches up on the back wall.
(actually on the top rim, but I am subtracting the gravel layer from
the total height of the tank. The output of the 404 is actually
behind the bio-wheel output, which is a waterfall type output that has
plastic extending about 1.5 inches below water level, so the 404
output nozel rests completely behind this 'plastic' wall and is
directed to the right hand corner... thus giving me fairly good round
the tank circulation.

The tank is very highly oxygenated... 3 airstones... so there are
those small bubbles floating around, just not very many by the intake
of the 404...

shoot... I don't think I can describe it any better... the top locks
down like it is supposed to... could it be the little hand-pump primer
on the 404? That doesn't seem to have any sort of tight fitting "O"
ring around the plunger, could the air be coming in there?

Obviously, I can't test it with soapy water since that would cause the
soap to be ingested into the filter.
Any suggestions?