A receiver has a automatic gain control. Then there is a strong signal the receiver turns down the gain and thus the noise goes down. Then there is no signal the receiver slowly turns up the gain until it finds a signal or until the noise is very loud.
If you keep sending something, often enough that the receiver keeps the gain turned down, then you will not have this noise problem. You can send, ..................data......................data.......................and in software remove the ".".