Are there any particular noticeable signs of a submersible well pump going bad. In the past couple of weeks, I've noticed that when it cycles on, I hear three or four "knocks" in the pipes as compared to just the one when it first kicks on, as it used to be. If I hang out by the pressure tank with my hand on the incoming line, I can actually feel these surges like the pump is actually going on and off a few times. I had something similar a few years ago when I had small holes in the galvanized pipe coming out of the well. In that case, I would get spitting when I had the water on, but that's not the case here. No spitting, I just get the noise. Also, I had the galvanized replaced with plastic back then also.
I'm not a pro or expert here, but have worked on a few systems in my life. The pump would be the last thing to suspect--there's no switches or contacts on the pump itself, so it will either run or not run. A possible exception would be a bad wire splice at the motor which has worked loose over time, giving an intermittant connection but this is unlikely if the system was professionally installed.
First thing to check is the pressure switch, which would be a plastic roundish rectangular box mounted somewhere in the vicinity of the tank inline with the piping. That's what turns the pump on when the pressure goes down to a preset limit, and off when the line pressure goes back up to 40 psi or whatever pressure limit the installer set it to. Pull the cover off the pressure switch, give a visual check for rusting or loose connections, and monitor it when the pump cycles on. The contacts may be badly pitted, or the mechanism may just be wore out if more than 15 years old. They're about $25-30 at a plumbing supply. I'd recommend a plumber to check it out and replace if necessary, since setting the pressure limits on the unit is kind of tricky I'm hesitant to give this advice--but if you are comfortable working with electrical stuff, you can use a plastic chopstick or disposable pen tube to hold the contacts closed right after the pump starts, just for a few seconds. If the pump does the same thing, the next thing to check is the control box which contains the start capacitor and relay. It's a rectangular metal box about 4" wide, 8" long, and 2" deep and there should be an electrical connection between it and the pressure switch. The cover pulls off (AFTER YOU CUT THE POWER SUPPLY) and you can take it to a pro to have it checked out on a meter. Some of them give instructions for doing this inside the cover, using an ohmmeter. They are about $50-60, not cheap but if just the capacitor is bad you can replace that for about $10. That's a black plastic cylinder with wires connected to it.
Otherwise, if your pump is older than 15 years it's nearing the end of it's rated life and getting time to think about replacing it.
If you don't have more than a basic familiarity with electrical stuff and don't know how to protect yourself from electric shock, disregard all my advice and call a pro to check out the pressure switch and the control box. A well pit is one of the most dangerous places to do electrical work because it's so easy to be grounded at any part of your body. In fact, disregard it and call a pro anyway--I'm just telling you what I would do.
One of the things that would check is the circuit breaker itself, which powers the pump. A breaker which is going bad can deliver erratic voltage, and cause problems. You would want to check the voltage coming off the breaker while it is under load (the pump on). It may be easier to just replace the breaker, if you are able to do that yourself.
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