Related to problem on the BNC connector

I am using the BNC connector to feed the voltage to my Opamp Circuit as an anlog voltage ranging from + 500mv to -500 mv …I am using CA303 Opamp …and using the opamp as a buffer amplifier…I had put 1Mohm resistance in series with the BNC connector input to Opamp Pin No 3 …I had also put 1nf capacitor accross the BNC connector postive and GND pin…There is 50mv noise that I am able to observe at frequecy of 11 Mhz when selector switch of CRO is kept at 10mv position and if I control the Voltage selector switch of CRO to higher position to 50mv…I am getting the same 50mv noise with 1Mhz… How to identify the correect frequecy of the noise from CRO? …Can any body recommand any filtering circuit for such noise reduction?..

One thing I had attempted is I had grounded the Positive pin of BNC connectors gives me a stable noise free voltage and we are then measuring the GND to GND voltage… Is it possible that such noise can exists in GND itself ?