N Cook wrote:

2 of the same fan blowing air in over the heatsink but there is a noise like
a variable direction wind or slightly gusty wind as though one fan has a
bearing problem.
It would seem one fan is fighting the other to some extent, significant ? or
just one of those things ? .
I was thinking of putting a dividing baffle between the 2.

Disconnecting each in turn and they run perfectly normally with no change of
sound. I may dig out a strobe if i can remember where it is and see if the
blade speed does vary in the double active case.

Beat oscillation. Basically, the normal fan physical noise pitch of the
two fans are _almost_ the same. What you're hearing is the difference between
the two. (it may be less than 1Hz, because the fans will probably be quite
closely matched in noise and speed).

It doesn't cause any harm other than you getting fed up with the noise and
smashing it to bits.

To "fix" the problem, you could try to physically isolate the two fans from
each other (or the common chassis) by using some sponge or rubber or some
such, to help prevent fan vibration getting into the chassis or other fan (if
you have them physically stuck together).

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