Interference on C band

C band interference is a possibility.  The same frequencies may be used for terrestrial radio relay links and possibly new C band services ref: https://www.satsig.net/itu/c-band-spectrum.htm   There is also the possibility of interference from radar systems which may be on close by frequencies but at very high power.    Can you see any obvious source of interference such as radio relay link tower, airport radar etc.  

The only way I know to definitely diagnose this problem is with a spectrum analyser.   You might try a simple satellite signal power meter and connect it to an LNB and feed and then try holding the LNB/feed and pointing it in all directions around, and straight up for a cold sky reference.   If the meter is sufficiently sensitive, you will see the warm ground if you point it down or at walls/trees. You can walk around and measure the C band radio noise coming from anything. Do the same in some radio quiet location and compare results.

If it really is interference you need to hide the dish like so: https://www.satsig.net/satellite/reducing-interference-satellite-tv.htm

I would also consider the possibility of some misunderstanding regarding frequency, polarisation, pointing etc.  Is the LNB actually working properly?.  Does the LNB and modem work normally at other locations ?   Is the dish correctly assembled (e.g. upside down or wrong feed strut arm holes ?).  Is the satellite direction obstructed by overhead power wires, walls, trees ?  Is there an earthing problem such that mains voltages occur between the roof metal work and the indoor modem earth ? Take care.  A spurious stray 30 to 50 volts AC could give you a shock and also damage LNBs.  Use a thick earth cable between the dish structure and the indoor modem rack earth.  A thick lightning earth is also recommended but beware stray mains voltages between the real earth and the building metal !
wxw
Best regards, Eric.