Ticking noise under load, can also be Detonation/Pre-Igntion, but its usually louder than a "tick". Sounds more like pop corn popping, higher octane gas or solving the root cause of the detonation/pre-ignition is the cure for that.
It can be valve tick, even with lash/self-adjusting lifters. The lifters get gummed up over time/use and fail to adjust proper to take up the valve lash. Giving you ticking in the valve train. This usually happens at idle or low engine speed, with the noise disappearing at higher speeds, but its possible for it to happen the other way around. Usually higher oil pressures get the lifter to respond and the low oil pressure at idle/low rpm let it bleed down. But you can have a slightly collasped lifter that won't extend at all, even under higher oil pressure, and it has very little lash, so the noise isn't noticeable until higher engine rpm. The easy answer for this is too try some of the additives that is suppossed to clean out varnish gum out of the engine and remove lifter/valve tick. If it doesn't work, either replace or remove/dis-assemble/clean the lifters.