From looking at the blow-off valve in question, it's pretty clear that the vendor really wants you to hear anything that the valve is doing. It's venting overboost to atmosphere through a little trumpet:
Based on your description, my suspicion is that you're hearing the piston (shown in the image) partially opening the vent port. This could happen in a situation of part throttle / partial boost situation that's sufficient to cause the piston to move but not quite enough for the classic FFFFSSSHHH blow-off valve sound.
This doesn't sound like compressor surge. In that case, you'd hear a rhythmic repeating chirp (or other descriptive word) after slamming shut the throttle.
You should be able to reproduce this situation with the help of a friend:
With the hood open, have your friend open the throttle wide and quickly slam it shut. You should hear the immediate whoosh of a full blow from the valve. NOTE: For those who understand turbos, I know that the wastegate usually won't let the turbo spin up if the engine isn't under load. Enough boost should be generated to trigger the valve, though.
Once you've heard the full blow, have your friend try progressively decreasing throttle settings. I suspect that you'll eventually hear the whoosh turn into the gurgling sound that you describe where the piston doesn't fully open.
If neither of those work, try modulating throttle while driving in too high a gear. There won't be enough exhaust volume for the turbo to reach full speed. You might be able to find the exact setting where the piston partially opens.