XPS14Z fan always on with low CPU usage

I have a XPS14Z (Win7 64b) and I have recently changed my HD to SSD (Samsung 256Gb). After that the fan was always on and noisy. I cleaned the vents and it fixed the problem: fan often off and very quiet when on. However after a few days I had to replace the bottom cover and since then the fan turns on a few minutes after booting, it never stops and it's noisier than before.

I have tried to remove the bottom case and put it back on, but still same problem (actually the fan spins also when the bottom case off the laptop...).

I have updated the BIOS to the latest version (A06 for Win7 64b).

Core temp shows 53°C after 30 minutes at around 10% load. 

Dell pre-boot diagnostic check shows no problem. High speed is 4400rpm and low speed is 3500rpm. Speed is generally around 4000rpm. Is it normal? Why is it noisy still at minimum speed?

It seems unlikely that I have to change thermal paste, because two days ago the fan was silent and not always on... can you please help me?