Hair Washing Gives a Tinnitus Spike?

I believe it could be due to senses in change in temperature + massaging the scalp aggravating the auditory cortex - the part of the temporal lobe above each ear that processes auditory information. It basically builds up its own pressure in response to the changes in water temperature, massaging, and chemicals in the shampoo, plus the skin on the scalp 'shrinks' slightly during hair washing putting even more pressure/stress on that area.

Avoid leaning over the bath or leaning forward which raises pressure in the cranium and sit or stand in the shower instead. Also, try getting the temperature of the water as close as possible to the surrounding room temperature before washing. Avoid use of chemicals in your shampoo and if needed avoid shampoo altogether.