I have the same problem with the fan on for a long time with no apparent reason. My MacBook Pro is 4.5 years. I checked the Activity Monitor and there was no excessive activities from running program. When I upgraded my Mac and have to transfer data from the old MacBook Pro to a new one, the IT tech told me the battery in the old MacBook was buckled and the trackpad was leveled with the body of MacBook. The IT tech told it to an Apple Store to get a quote to replace the battery. I was told the battery was at the early stage of “melting” and glued to the bracket inside. I have to replace the entire battery and bracket and will cost around $500. I was also told that I was carrying a potential fire hazard ……..