HTC One M7 :: Frequent And Irregular Static Beep / Blip While On Call

I have an HTC One Dual Sim and while the phone is great, have had stellar battery as an added bonus, there is this very annoying static "pop" or "blip" whilst speaking on the phone. It is not regular, but very frequent. More often than not, a word or part of a sentence gets eaten up. Almost like the notification light can be heard as it flashed, but I checked this and there were no messages, mails, IM's or anything coming into or out of the phone.

I changed the phone thinking there may be an anomaly, but the 2nd one does the same thing as well..

Does this phone have the option to turn off in-call notifications, I can't find the setting.