could be a wheel speed sensor got some water intrusion. Checking for a fault stored code with diagnostic software would be helpful in ruling out or confirming. It would also specify front or rear. If it stops once things dry out, which sometimes happens but not always, would be a clue if it is indeed a water logged sensor. A hair dryer on low heat on the wheel sensors is worth a try. Follow the wire leading to each in proximity of the tone wheels on front and rear. A little silicone sealant around where the wire enters the plastic sensor is a good idea even on new sensors. Best of luck , dave
I thought about the wheel speed sensors but they are a digital device, certainly if one gets moisture inside they will trigger the ABS fault light, its the half on that puzzles me, thats more like a leakage.
Just dry any connectors off, WD40 might help, the WD stands for Water Dispersant.
As lucian says the wheel speed sensors will suck in water, it enters between the soft cable sheath and the hard epoxy body, you could try warming the sensor up to see if that improves it, The first thing I do with these sensors is smear a little silicone sealant around the point where the cable enters, the silicone provides a flexible seal.
Guzzi drag may throw up a fault code I know it did for the sensor on my friends 1400.