Impulsive signal during a USRP receiver (without data transitter )

for my thesis I have to simulate some modulation (QAM, PSK …)using two
USRP. But first of all I would check interferences present in the
environment at determinate frequency. So I do a file .grc with 2 blocks:
USRP_source connects at bolck fft_sink. My problem is that when I change
the freq of the USRP I always see a impulsive signal of about 34 dB. In
some case there are other signal around my central frequency. When I
unplug the antenna all the signals fall down except the signal at
central frequency of 34 dB.My question is: does this impulsive due from
the inner VCO of the USRP or I have something wrong in the blocks?

I attach two snapshots, with and without antenna.