1992 Toyota Celica distributor replaced, mileage drops 130

1992 Toyota Celica 4 cyl Two Wheel Drive 134000 miles

Driving home, my steering column tightens up while there is a simultaneous loss in power, the car conks out. New distributor is needed, and my mechanic installs a $220 aftermarket distributor; the OEM is closer to $600 in cost. Would this cost difference result in a change in mileage efficiency? Went from 35/40 miles normal/highway to a constant 28-30. I also noticed the speedometer needle has taken on a noticeable fine tune vibration while accelerating and at constant speeds. There is a faint (have to move ear pretty close to the dash) noise coming from the dash. It sounds like fast a ta. Ta.-Ta-ta-ta-ta-ta that slows and quickens with car speed. Lastly, and also after the distributor replacement, when I first start the car there is a distinct 3-4 second whirring/hum noise coming from what sounds like under the car passenger side. The whirring happens less often as the weather has gotten milder. Had it back to my mechanic but he seems stumped on what to do next. All filters, plugs/wires and timing look good. I dont mean to be redundant or draw attention away from other possibilities, but the mileage was fine before the replacement. Thanks for Your advice.