Boiler gas valve solenoids clicking ?

Potterton Netaheat Profile 80e boiler about 20years old. I have just replaced the gas valve including the two solenoids after noticing the solenoids were occasionally clicking rapidly in short succession. When normal I hear the solenoid clicking once or twice to start and shut off the gas supply. Following the gas valve replacement there is still an occasional series of short (approx 15sec) rapid clicks from the solenoids and the burner continues to operate then the clicks stop with burner still is operating. A few minutes later the solenoids operate correctly and shut gas burner until called again on temperature drop.
Any ideas what is causing the unusal short burst of rapid continuous clicking of the solenoids?