I don't believe it really matters whether you are talking about the BCM50 itself or the Expansion unit.
It's likely a matter of WHEN the unit was manufactured.
Nortel likely used several different Asian vendors to provide those tiny fans over the years.
I am, unfortunately, a recipient of one of those noisy fan Expansion units. Mine was manufactured in late 2009.
So, of course, I made it my mission to investigate the replacement of those pesky fans.
I have found that "Nortel Authorized" dealers tend to sell them for a rather outrageous
$40+ a pop. $80 to replace both fans? Come on!!
At that price, it would be more sensible to replace the entire Expansion unit
(not the Media Bay Module, however).
I have found the fan for close to $20 at one supplier,
but that still seems a lot to pay for a $2 to $3 fan unit.
It would take some time, and an understanding of what all three wires coming from the
fan do (I am assuming ground, power and speed sensor).
The connector seems like a common type.
So that is the extent of my experience on the matter.
Anyone else have some further data?