I've had an issue in a condo I manage for a couple of months and after much trial and error, I thought I'd check with BP to see if anyone might have an idea what might be going on.  The condo is a newer unit (2004) on a first floor.  My tenant has been complaining of an occasional knocking sound coming from the upper half of an interior wall.  We have run all the water in the unit and in the unit above it continuously, flushed toilets, etc. and have not found a correlation as to when the knocking comes.  It also does not seem to be tied to when the furnace powers on and off.  It happens more during the middle of the night than daytime, but daytime knocks happen too, and are getting more frequent.  It is a single loud "knock" vs. a tapping.  My plumber says that he would have to open up a wall to see if he can figure it out.  I really am not looking forward to going that route, as I'm thinking there is a good chance he would not find anything.   Thanks for reading!  

How much would a wall have to be opened up?  Is it possible to just drill a small-ish hole and send in a camera?

@Dawn Anastasi - interesting idea!  Thx.  

possible a small creature?

@Jonna Weber 

I wish I knew the answer. I want to guess that it's an expansion type of knock.

But @Dawn Anastasi might be right, these small manipulative scopes can do wonders and save tons of heartache.

drilling a small hole for the camera would be the cheapest route. If you do not find anything you won't even need to patch it when your done. You can just use a blank outlet cover plate. 

Thanks all - I am having my tenant put a piece of tape on the wall when she hears the noise....trying to narrow down the exact locations. It has been a goose chase, and we are also trying to determine if it is a plumbing or HVAC issue. I'll let you know what we figure out... :) 

I have had the tubing from outside A/C unit going to the inside unit do that.  There was extra tubing that was coiled and when the compressor kicked on it shook the tubing.

@James Sinclair   - Interesting. Were they using the A/C at that time of year?  They are only running the furnace right now.  I think I will call my HVAC professional and talk it through with him.  Thank you!  

Is it weather related?  We have a fairly new house that has downright scary loud knocks when it gets very cold out.  We were told they were some sort of "expansion/contraction" issue.  It only happens when it's very cold though, no other time.  We were told it's normal.