Mitsubishi Colt CZC problem
 

Hi needing some help, had my car to two different garages had diagnostics twice and car recently passed not but there is nothing showing up to indicate a problem. However after damp nights or rain through the day when I start the car I turn the key and it is slow on turning over it sounds like the car is sluggish on starting. The only way I can describe it is that it sounds like a swishing of water noise like clothes in a washing machine. Not a hum, or a squeel or spinning but clothes going around in the water. I then have to pump the accelerator pedal a couple of times or slightly press clutch in for it to fire up faster to start properly. Battery is fine, alternator is fine. Only 52000 miles on clock 2008 convertable. Any help would be appreciated as garages can't find it.
Thanks in advance