My central air throws out a musty smell when in use do you know what is causing this

My guess is that there is water standing near the evaporator that is growing some mold or something.

The evaporator is the coil located in your furnace unit. During operation, water from the humid air is condensed on the cold evaporator and falls into a pan below it. There is a drain pipe connected to this pan and it may be clogged up. Sometimes it is easy to clean the clog, sometimes it is difficult. Hopefully someone more experienced than I am will chime in with a good method to do this.

Best of luck,
WallyH