Like HMX says, you can hear them run, an efficient whjirring, not rough. They run in series to make the needed pressure, seems like its eighty-something PSI. I replaced both the pumps on my car cause it wasn't running when I bought it. Be sure all copper washers are in place and tight, also hoses are good. With a slow leak you can smell gas but it evaporates before it puddles. Pump relay can keep the pumps off, too. Good luck!