My CPU fan makes a slight 'clicking' sound sometimes and then its fine

Alright so one day when i turned on my computer i heard a smaaaall amount of 'clicking', i you have to put your ear close to listen, it sounds like it is spinning against dust or a spec of something. And i did clean around the CPU fan carefully to get rid of the dust mites. my computer case has alot of ventilation due to the fact its a Antec 900. so it doesn’t seem to overheat at all but the CPU fan has me concerned because it does the sound then it stops and goes normally like a peaceful humming and also, sometimes if i turn it on it DOSENT click. i also tested this theory and found out if i put bit of pressure while the fan clicked the sound stopped and if i let it go it continued until normal. what are your opinions on this Phenomenon?

PS. my knowledge of computer hardware is a little medium so go easy on me