it could be frame flex under load
it could be your chain is getting too worn

It might also be that you need a new derailleur cable because it is losing tension.

Check to see if you can just tune up the indexing without replacing the cable:
(instructions for a triple
1. Shift chain to middle chainring in the front and innermost (largest) rear sprocket.
2. View gap between inner cage plate and chain. Gap should be as small as possible without rubbing chain.
3. To reduce gap, increase inner wire tension by turning barrel adjuster counter-clockwise. Adjusting barrel may be on frame or on shift lever. Check gap again.
4. If chain is rubbing cage, turn barrel adjuster clockwise, or inward toward shifter body.
5. If barrel adjuster is all the way in or out and no adjustment is possible, reset inner wire tension. Shift to innermost chainring and loosen inner wire pinch bolt. Pull wire gently with fourth-hand tool and tighten pinch bolt. Begin adjustment of inner wire tension as above.

I'd expect that you'd find you need to tighten the cable. If tightening the cable causes problems with rubbing, you might need a new cable and/or chain (i.e., either the cable is just failing on you when set properly or the cable isn't your problem at all). If tightening the cable causes no problems, then check the indexing by making sure that you can still shift into the other rings, particularly the little ring. It is too tight if you cannot shift into the little ring on purpose. It is too loose if you have slow shifts into the large ring.


it could also very well be your cross-chaining.

When you're in a fairly hard gear under heavy load, there will be a lot of tension on the top of the chain, and the chain will be kind of slack on the bottom. this can cause the chain to jump in all kinds of ways, especially on a flexible frame. The most extreme is that in a sprint effort, the chain can jump into your spokes. More commonly, it will drop rings. You can avoid it a little bit by picking a gear where the chain is straighter or by trying to spin more in an easier gear.

If you work on the cables (including replacing them) and follow your friend's advice and it is still a problem, then most likely, you're due for a new chain. When you check the chain, you should also check the cassette for wear.

That clink you heard was probably the chain rubbing on the derailleur from being in the little ring and a small cog.