If have been beside a 1250 MVAR unit when synchronized to the grid. No discernable sound when it synchronized. The shafting was painted with black and white stripes at one point, and under the discharge lighting in the turbine bay, the difference speed prior to synchronization could easily be observed. The shaft was rolling very slow above synchronous speed, and when the breaker closed, the difference in speed stopped. As load is added, I could easily see the phase angle shift a little.
What does make noise is when the phase angle shifts due to grid disturbances. Being a fairly big machine there is some flex in the shaft and it made funny noises, in combination with the sound GIANT steam valves closing and re-opening trying to damp the oscillations.
A trip under load did bang, but that was the steam valves snapping shut plus the breakers. The breakers were opened and closed using 450 PSI air. When opening the breakers did make a pretty big retort, even though there was a muffler. The cylinder was about 6" by about 3', so that's a LOT of air to vent when charged to 450 PSI.
The breakers would have PM after a certain number of cycles or years went by. I believe that usually it was the time limit that expired. I don't think the contacts were ever replaced, but that wasn't the area I worked in. The responsibility for the HV breakers were under the substation maintenance department of the company, far removed from plant operations.
Figure 50 to 200 MW for gas turbine combined cycle plants. Figure 300 to 750 MW for coal fired plants Figure 600 to 1400 MW for nuke plants. Hydro anywhere from the kW range to 600 MW Wind turbine from the sub MW to 10MW, but most new in the 2-5 MW range.
I don't think they used grading resistors to cut the lines in and out at first so it may have well banged because of the current inrush when the line closed in. I have only once heard a 500kV line energize (the highest we have on the west coast, at least for AC), and it had grading resistors to energize the line. Lots of buzzing as the arms went by each resistor, but I didn't hear any "bang" from the line.