whistling valves

[BDI123]

I have a Miraphone 1291 that has developed a whistling when just pushing the valves down. It has me baffled, has anyone else run into this,the valves are always oiled and I don't know where to go from here, it is very annoying though.

[PlayTheTuba]

Maybe valve oil is entering the breather hole at the top and not properly exiting through the hole on the bottom?

[GeoffC_UK]

Maintenance, may be?
Dismantle each valve completely, including bottom and top caps, springs, etc.
Soak in warm water and add a brass soaking agent to help things along.
Make sure any holes are clear, such as the bottom cap and breather hole through the valve itself.
PlayTheTuba, above, quite rightly mentions the breather hole being blocked - flush this through in your warm bath.
Rinse each part in running cold water, afterwards, then dry with a lint-free cloth, before oiling and re-assembly.
Avoid covering any holes with rubber spring dampening washers, too.
Make sure to return all valve parts to the same valve casing.

[TubaBeage]

As Geoff says it's most likely the valve breather holes, usually obstructed by a wet top felt.
Make sure you get the water out of the valve pipework before you store it after playing.