Unfortunately, after a few days trouble free motoring, the “medium frequency buzzing” I described in my original post has returned, this time with no battery in the mic and with it operating on phantom power.
It is clearly an intermittent issue. It only occurs when input echo is engaged. I don’t have another mic, or even access to one I can borrow to try to establish if it’s the mic. Ditto another audio interface. I have changed the mic cable, so it’s not that. It’s not related to any heavy duty household appliances being used. I don’t think it is an earth hum (frequency too high). It doesn’t occur when I plug in an electric guitar.
I live in Spain and it is tempting to blame dodgy household electrics, yet my gut says this isn’t the case, as it hasn’t been a problem before in the two years we’ve been here. I am beginning to think it might be the mic itself.
Any pointers as to how I can isolate the issue and discover the cause?