I replaced a hot water heater which was already wired to a 2 pole 20 amp breaker. Recently the main started tripping, so I replaced the main breaker (100 amp). Now the water heater is buzzing and has some sparks in the back of the breaker. It is a mobile home (1991 redman) and has a white westinghouse breaker box. I have replaced the breaker also a few months ago, it is a big breaker with 30 amps on each end and 20 amp in the middle. I don't know came that way. The breaker is turned off for now, but we will need hot water tomorrow. Any ideas??

Honestly, unless you are experienced with this work I would call someone out immediately.
It sounds like you need a new panel, unless a savvy electrician can find an interior for yours.