Neumann M147 low freq popping problem.

So our M147's developed a fault in a session today.
It's started making a low ticking/popping noise after about 3 hours of use.
heres a sample.
http://www.sonicastudios.co.uk/old_files/m147pops.mp3

I was just recording a voice over (using a big foam pop shield) so it wasn't being abused at all.
Our vocals booths air conditioned and the studios dry, so i really don't think its a heat or dirty/moist capsule sort of problem.
I've read a few threads on the web indicating it could be the valve? . Plus, for a M147 it is fairly "old" The serials <50.
Anyone here have any ideas?
I'll set it up in the morning and try and recreate the fault.
thanks paul

Is this the mic which used an underheated sub-mini which shouldn't have really been underheated?

If so, might be worth trying to swap out the valve, or cook the current one in a test circuit at rated heater and a decent amount of dissipation (say 25% or something).

This is the one with the 6111 sub-mini dual triod.

http://www.gearslutz.com/board/attachments/geekslutz-forum/230113d1302683928-replacement-tube-m147-neumann_m147.jpg

My friend's 149 had this problem along with some crackling and hiss. Unfortunately these tubes can be noisy so unless you want to buy a bunch and audition them, neumann sells the tube and daughter board with the rest of the output stage that the tube is glued to for around 350 usd iirc.

Thanks guys,
I've found a Russian equivalent tube (6N16B) that's cheap, think ill buy a bunch before i go to Sennheiser for a "selected" 6111.(€200)
I read that the Russian military tubes are more consistent than the 6111's.
http://www.gearslutz.com/board/geekslutz-forum/599223-replacement-tube-m147-2.html

I left the mic on all day yesterday, in the morning the problem was still there, by the afternoon the popping had stopped but it was a little noisier than usual i think. Anyhow it won't be used again until serviced.

So i replaced the valve with one from Watford Valves and the mic's back to normal.
I'm assuming it was a tested/selected one @ £20? as the mic's now fine.
http://www.watfordvalves.com/product_detail.asp?id=125

Loads of space inside the mic body for mods, actually would be very easy to replace the electronics.
Just need to desolder two pins for the capsule and undo one screw.