"4G as in mobile data?" I hear you ask. Well you are right.
The old analogue channels 1-4 were terminated (digital switchover) to allow 4G / LTE to expand throughout the UK. Whilst 4G / LTE is great for your mobile devices, those frequencies are very close to those that carry our Freeview channels. In some cases if there is a 4G / LTE mast between your TV aerial and the transmitter that you receive your channels from, it can result in a blocking picture and screeching/clicking sound.
Even though 4G / LTE has been around for a while, there are masts popping up every week which could then be the start of your TV viewing nightmare.
Please remember that 4G / LTE can only interfere with TV channels that you receive through a TV aerial (DVB-T & DVB-T2), and not a satellite dish (DVB-S & DVB-S2).
Here is a picture of 4G / LTE interference present at 800mhz: