Oil Boilers: Startup Noise
A banging noise, accompanied by a sooty puff back when your boiler starts up, might indicate a problem with the oil pump. Oil that has not burned completely may be leaking back into the combustion chamber after each burn cycle; this oil then ignites when the furnace starts, causing a loud bang and a cloud of soot. If this is the case, immediate repair is recommended. If your boiler makes more of a rumbling noise when it starts up, it probably means the system needs to be inspected and cleaned.
Oil Boilers: Shutdown Noise
A rumbling or stumbling noise, after your oil boiler shuts down, is generally indicative of the same problem that causes a bang when it starts up. In all likelihood, oil is leaking back into the combustion chamber.
Oil Boilers: Pipe and Radiator Noise
Clanking and snapping noises in your pipes are normal. These sounds are caused by metals expanding during the start of a new heating cycle and are most common if your boiler has not been used in a while. Radiator hiss is also normal but is more common with steam boilers than with oil-fired systems. A bubbling or rumbling sound may be caused by excessive air in the pipes, in which case the unwanted air will need to be bled out.
Oil Boilers: Shrieks or Grinding Noises
Shrieking or grinding noises in your oil burner or circulator pump indicate a serious problem -- possibly a failing bearing -- which requires service as soon as possible.
Steam Boilers: Banging Caused by Improper Slope
Banging, clanging, knocking and pounding sounds -- which many homeowners describe as sounding like someone in their basement is hitting their pipes with a hammer -- are caused by a number of problems. Most can be traced back to drainage; condensate accumulates in pipes rather than returning to the boiler, causing a phenomenon called "water hammer." This is often the result of the pipes or radiator situated at such an angle that water cannot flow back to the boiler as it should. If the radiator is the problem, you can add shims under the end of the radiator opposite the steam pipe. If pipes are the issue, you may have to either live with the problem or call a professional.
Steam Boilers: Other Causes of Banging Noise
The clanging and banging sounds in your basement and under the radiator may have a few other causes. Air can sometimes become trapped in the pipes, causing a sound similar to water hammer. Condensate can also be trapped by a partly opened radiator valve. A worn or damaged valve seat could also be the culprit, and banging has been attributed to various broken internal parts.
Steam Boilers: Boiler Noise
Banging noises in the boiler itself are typically the result of poor maintenance. Sludge accumulates in steam boilers over time and needs to be drained periodically. In most steam boilers, the water cutoff control has a drain valve that can be opened to drain excess hot sludge water. Consult with a professional who can show you how to do this safely.