Initially during high processing loads up to recently including basically all powered-up time my MacBook Pro exhibited a "death rattle", an intermittent, staccato, sometimes quiet sometimes loud sound not unlike a muffled version of playing cards in your bike tires. Sound was intermittent for months and seemed not to correlate to anything in particular (position of laptop, etc.)
Bing bang boom. This is a very simple repair if you observe basic opening-my-case protective measures.
I looked at every possible support page, blog, etc. as this was ongoing and intermittent for months. Grounding contacts ticking against backplate, various battery/smc related stuff, etc. The diagnostic that indicated the fan was ultimately taking a close look at RPMs in SMC fan control when the ticking started ... there was a noticeable downward trend in RPM when even a slight ticking would start. This is my recommended diagnostic.
The fan for this model appears to be made so quiet by some sort of magnetic system in the spool of the fan. On close inspection the housing on the underside of the spool was ever so slightly angular and not flush with its casing. My speculation is that over time (mac is now 3 years old) this very slight angle caused a vibration, then a wobble, and ultimately contact between the fan blades and the bottom plate of the fan. Very, very fine plastic dust was present all around this area, and the fan blades themselves were ever so slightly "flattened" by the high RPM contact.