Intermittent Sennheiser Fault

[paulears]

See if you can borrow an analyser - there a re a few quite cheap ones now, so somebody local to you may have one? They'll clearly show you what is going on locally. Two string signals below your operating frequency can produce all kinds of weird spurious outputs. A faulty transmitter, especially a higher power one can also produce a field of outputs on mathematically linked frequencies. They tend to die away quickly with distance, so sometimes go undetected. Worth a go too with a simple scanner - with no radio mics on, scan through the band and see if you hear anything.If you do, but can't work out where it comes from, see if there's a local ham radio club - their members quite like mysteries, and will almost certainly have the kit to track it down - and see it as a bit of a challenge.