Was wondering if anyone could explain where this annoying sound may come from? Looks like it comes from the heater/ventilation air-duct inside the wall, a few feet up from the opening into room. Whenever the heater and fan are on for a few minutes and the air coming out is warm, it starts and goes on as constant, evenly spaced, a minute or so apart, "knocks" or "clicks" or water "drops". When the heater and fan are turned off, the "dripping" doesn't go right away. The time between successive "clicks" becomes longer and longer before it gradually disappears after about 30 minutes. (Is something cooling down?) This sound is loud enough to wake one up at night. It is similar to the sound made by a drop hitting a floor from a hight or by a metal wire hitting a board. Any suggestion would be greatly appreciated.

UPDATE and SOLUTION: After closer examination the pattern of clicking was a bit different. The sound always started when both heater and fan are on for a minute or two, clicking once a second or so, then it slowly disappeared. When the heater is off, with the fan going on for several more minutes, the sounds starts again, a click per second, and then slowly disappears just as in the beginning.

It had something to do with first heating and then cooling down of the connection of two tin air ducts where they are attached to each other at an angle. It seems that some loose metal parts within this connection and metallic tape around it made this effect when they were warmed up or cooled down. This connection has never made any sound when it was cold or when became hot -- only somewhere in between. Who would think.

I have the same problem, not sure if it's a problem or normal house noise, but it's annoying me enough where I searched the web for a solution. I'm going to head up to the attic tomorrow to see if I can tell what's causing my noise. Doesn't happen when it's blowing cold air, just hot, so I think this has to do with thermal expansion of duct and/or supporting members like wood joists that link up w/ the duct. But that's just my initial guess. I'll see what I find and try to post results.

Have seen it and have fixed a few of them:

Symptom: Continuous random click and knock noise from one or more heating duct, most often on those running vertical to 2nd and 3rd floors. On a furnace with a DC motor, while the motor is on but furnace not burning, you will not have the noise. When the furnace comes on, you will start hearing random and continuous click and knock sounds that vary in intensity and frequency (20 to 40+ decibels, and 2 to 30+ seconds apart). As soon as the furnace goes off, the click and knock stops.

The cause could most likely be:
1. a duct that is not properly secured with straps (thus allowing it to bend out of form more than the others), even the straps can expand, or
2. a loose duct fitting, or
3. dents in the duct that expand and retract right away.

Most warranties do not cover expansion and contraction. This type of noise falls in the gray area, as it's onset is caused by expansion, but it is due to improper installation or damaged ducts. But how can you prove a duct inside a mechanical wall is damaged or not properly installed.

It MAY BE POSSIBLE to correct this by cutting the drywall and making some adjustments to the duct work. Alternatively, use a damper to limit the airflow to the affected rooms (then the rooms will be cold). Even if you cut up the drywall, you may not be able to locate the problem as the expansion/contractions are less than 1 mm, and could be on the side facing the floor or the .

LESSONS LEARNED FROM MY PREVIOUS JOBS: Before you drywall, run the furnace and listen for unusual expansion and contraction sounds and fix them before you close up the walls.

Have seen it and have fixed a few of them:

Symptom: Continuous random click and knock noise from one or more heating duct, most often on those running vertical to 2nd and 3rd floors. On a furnace with a DC motor, while the motor is on but furnace not burning, you will not have the noise. When the furnace comes on, you will start hearing random and continuous click and knock sounds that vary in intensity and frequency (20 to 40+ decibels, and 2 to 30+ seconds apart). As soon as the furnace goes off, the click and knock stops.

The cause could most likely be:
1. a duct that is not properly secured with straps (thus allowing it to bend out of form more than the others), even the straps can expand, or
2. a loose duct fitting, or
3. dents in the duct that expand and retract right away.

Most warranties do not cover expansion and contraction. This type of noise falls in the gray area, as it's onset is caused by expansion, but it is due to improper installation or damaged ducts. But how can you prove a duct inside a mechanical wall is damaged or not properly installed.

It MAY BE POSSIBLE to correct this by cutting the drywall and making some adjustments to the duct work. Alternatively, use a damper to limit the airflow to the affected rooms (then the rooms will be cold). Even if you cut up the drywall, you may not be able to locate the problem as the expansion/contractions are less than 1 mm, and could be on the side facing the floor or the .

LESSONS LEARNED FROM MY PREVIOUS JOBS: Before you drywall, run the furnace and listen for unusual expansion and contraction sounds and fix them before you close up the walls.