After shower has been running for a couple of minutes there is a loud train coming through noise and loss of water pressure. If I turn off water and then back on it goes away for another couple of minutes

You have a washer vibrating in the shower system. Start by removing the head and see if the vibrtionn/noise goes away. It could be in your mixing valve and, in that case, the noise will continue. Back to you

If this is an older screw style washered valve, the washer screw most likely came loose. Not uncommon, have seen it 2X this year. .