I had to use once a Schoeps mic which did same noises, I think. It did whistleing noises and occasionally there was a noise like someone was tapping on the mic. Problem was that something inside the mic was a bit faulty and it needed about 10 minutes to "warm up".
I really do despair of Modern Yoof who have all this tasty gear all relying on good leads and sometime batteries who will not invest in the most basic of test meters such as this.. ANALOGUE MULTIMETER: Amazon.co.uk: Electronics
And take an hour or so to learn to use it!
Hooked across the OP's XLR plug this will tell him very quickly if the spook juice is within specc' (48V +or -4V) and also if it is wandering or jumping about (analogue meter being better for this task than digital).
I however, suspect a dry joint in the mic lead screen path.