The cellphone here is the prime suspect for a weird thumping the
TV set is doing occasionaly...
Typical cell phones are in communication with the base station even
when you are not using the phone for conversation. This is how the
cell phone company "knows" which cell you're on in case you are
moving. (They do not ring every cell tower in the world. That would
be a waste of spectrum.)
There are "channels" for signalling, and "channels" for voice
communications.
If nothing changes, your cell phone might stay on a single signalling
"channel" for weeks.
But...
If your phone experiences a fade, or loss of signal, the reacquisition
may occur on another signalling "channel". (This can, and does, often
happen many times a day.) It can also happen if the cell site
transmitter drops the channel, or if they dynamically reallocate
spectrum to deal with cell site congestion issues, etc...
The point is, when your cell phone (lying peacefully on the TV table)
needs to get updated, it will broadcast a short burst of data back to
the serving cell site. This is probably what you are experiencing (TV
Interference) when the phone is not in Communication.
To extend this reasoning, when you are talking on the phone, First,
you are using a different "channel". One other than the Signalling
"channel", but very likely in the same general band. But many things
have changed. The orientation to the TV for one thing. (Presumably
you are further away unless you put your ear on the TV table?) Also,
the cell site may have instructed the cell phone to reduce output
power on the voice channel, etc...
Unintentional receivers in close proxmity to cell phones and the like
are usually bombarded with fairly high RF signal strengths, which
their front ends were not designed to deal with. While it is
certainly possible to make receivers that would be immune to this
proximity effect, it is not often economical, and may impose physical
size restrictions (for the front end filtering) which consumers could
find objectionable. In short, you just have to live with it.
BTW, I offset the word "channels" in quotes because in the strictest
sense, not all cellular systems (air interface standards) use discrete
channalized bandwidth, but the intent should be clear from the above.
-mpm