Water Pump Surge

We just spent five nights in a partial hook-up park. We used our own shower a few times instead of the public showers. Everything worked fine till the last night. When the shower was running the water pump sounded as though it was surging. I thought perhaps the tank was getting low, so I filled the tank but the problem persisted. Now, the bathroom sink faucet runs just fine as does the kitchen sick faucet. Normally, when using the pump there is just a steady purr. With the shower it goes louder, then softer. Water pressure seems to stay the same. Trailer is a Starwood 5th wheel, by McKenzie. Where should I start looking for the problem? Thanks.