First of all, you're not supposed to put your pickup close to the preamp tubes!!
Several things come to mind. The first is microphonic pickups. A microphonic pickup will squeal like a pig if brought too close to a speaker - the magnetic fields will interact. This can also happen if you get too close to the output transformer for the same reason.
You didn't mention if it just started happening after the tube change or not. One thing I know for sure is that the Model T doesn't like people that leave the metal shield off it's tubes. If you didn't put them back on, and it just started happening with the tube change, this is the most likely cause.