Whistles have been appearing in the top two rounds of hveto for the last 3 days. To get some better statistics about the troublesome frequencies the IMC VCO is crossing when whistles are produced, I grabbed the times of all the glitches hveto finds in the top 2 rounds. Note, not all the glitches in these rounds will be whistles, but the majority will be. I then looked at the IMC VCO frequency when these glitches were present and plotted a histogram (Figure 1). Whistles appear most often ~78.94MHz. 

I then started to look at other VCOs (regardless of where they are) just to see what their frequencies were when whistles appeared. I grabbed ALS COMM, ALS DIFF, ALS X, ALS Y and SQZ. I then made scatter plots of the different VCO frequencies against the IMC VCO frequency when whistles were present (ALS COMM - Figure 2, ALS DIFF - Figure 3, ALS X - Figure 4, ALS Y - Figure 5, SQZ - Figure 6, apologies for the odd rendering of the y axes). It looks like ALS COMM and ALS DIFF are happily parked away from the troublesome IMC VCO frequencies. The SQZ VCO also wanders away from the IMC VCO frequency range around 79.168MHz. Even though ALS X and ALS Y are many km away from the IMC VCO I still looked at them just in case it was interesting. These VCOs do wander the closest to the IMC VCO.