The most common problems with harmoniums are sticky keys, notes which play even when the key is not pressed, rattling noises reeds.
Sticky Keys
If a key sticks down after being played, it usually is rubbing against an adjacent key. I have often found when moving from one chord to another if I drag my finger across two keys, I can end up pulling one key into the other and they stick together. They are a couple of solutions depending on the exact cause, but they are usually easy to fix. Sometimes the board directly below the keyboard is too tight and presses against one or more keys causing them to stick. Loosen the screws on either side and see if that solves the problem.
Rattles
Sometimes when playing you here a rattling noise, or possibly a squeak. Most often the rattling is the top cover, either the whole cover or just the glass in the cover. Try removing the cover and see if the rattle goes away. If it does, then you’ve found the culprit. Hold the cover in one hand a gently tap the glass with the other. If it rattles in the cover you need to find a way to keep the glass from moving in the frame of the cover. Sometimes small pieces of paper or business card placed in the slot where the glass is held by the frame can prevent it from rattling. Make the pieces small enough that you don’t have to look at them with the cover in place. To avoid damaging the wood finish, it’s best to avoid using glue or tape to hold the glass in place.