It’s most likely external noise, you might need a filter:
FlightAware 1090 MHz ADS-B Bandpass SMA Filter | The Pi Hut
A filtered LNA would also work and also reduce the loss due to the coax adapter cable through the window.
New Product: RTL-SDR Blog 1090 MHz ADS-B LNA
(Filtering is much better than with the passive filter i linked before)
(You’ll need a bias-t or the rtl-sdr v3 dongle with builtin bias-t to supply the LNA with power)
If you want you can check what frequency the noise is at, do a scan when the noise is present and another one when it isn’t:
Do I Need A Filter?