I have been through a long journey of noises with mine.
Firstly, bearing check. By grabbing the different parts of the frame and wiggling try and see if there’s any movement at the pivot points, it should be pretty obvious. If nothing obvious then go to next step.
I’d say the first place to check is the articulation bushings at the seat stay/chainstay joint. All of the other bearings lasted well but these seem to wear at a faster rate. If you pop the shock off, take the bolt out of the centre of the bushing and then separate the chain stay/seatstay. They consist of two plastic top hats that sit in the frame and then a metal tube that slots into it with the bolt going through this. Inspect the bolts, if they have excessive wear (silver instead of black) then they are rotating in the metal tube which they aren’t supposed to do, but seem to sometimes. This can cause noise/knocking/play. If they’re fine then pop the centre metal tube out (it might be quite tight), if it comes out easily and there is room/wiggle around it then the bush is probably just worn and will need replaced, they are available from hotlines and come as top hats/inner tube set. If your bolts are worn then you will need to replace the bolts and the bush.
If you take the top hats out of the frame, clean it up with some IPA and check the inside of the frame to see that the top hats have not been rotating in the frame and wearing it away. Again this isn’t supposed to happen but can – that’s a warranty job for new rear triangle. It’s pretty obvious as a ridge will have formed between them and they will not fit tightly in the frame any more.
If this they’re all fine and the bearings all move easily with no play then it’s probably not the pivots. My bearings lasted 2 years but were pretty shot when I replaced them. The bushings I went through three sets and a rear triangle. I do ride quite a lot.
Bearings for the 2014/15 zesty/Spicy are 6902 x 6 (all main pivot points, two per point) and 6900 x2 (in the shock shuttle). I’ve bought a mix of enduro bearings and, most recently, Katec bearings off of eBay. Both seem fine. Just get the correct size in 2RS flavour. Changing them is fairly easy with a socket set, big ass hammer and a G clamp. Though you do need a bit of imagination and be careful not to ever press on anything but the outer race of the new bearings.
If not the pivots then it could be anything! Front headset lower bearing seems to go as seal is shite. Loose cranks cause a knocking sound too and a couple of months ago I spent days searching for a creak that I kept blaming pivots for and turned out to be a snapped rear axle.
Hope that helps.