We have a drip, drip kind of noise in the wall between the living room and bedroom. I do not believe it is a leak and it only occurs in the winter when the heat is on. We live on the first floor of an old, 3 family home that we purchased about 3 years ago. How do I find out what is causing the noise? Also, the walls are plaster and they are cracking, chipping and in some places bulging. What do I need to do? Funds are limited.

Noises like that are often difficult to find. An expanding pipe or heat duct that is fastented to tight to the wall or going through a to small opening often is the cause of this type of noise As it expands it rubs the wood and as it does makes this type of sound.
If you had a leak you would see water stains or wet areas caused by this leak. This is a very common condition with older homes as the pipe supports often fail and the pipes end up resting on the wood structure. Another cause is electrical wires that are run through the same opening as the pipes/ducts. The wire often wedges the pipe tight and as the movement occurs it causes this type of noise. Wires are NEVER to be run through the same openings as pipes/ducts but often are in older homes as tenents and past contractors such as cable or phone people use these holes as a method of getting from one room to another.

thanks, your explanation is helpful. Can you comment on my plaster problem?