About a month after I got my S15, the engine started developing a noise that is known as "VTC Rattle". It's a common problem with the S14/S15 SR20DET, the VTC Cam Gear wears out and produces an annoying rattle noise that sounds like a diesel truck. It usually happens on a cold start but progressively get's worse and happens more often even when engine is warmed up. It all has to do with engine oil pressure, as the noise goes away when you step on the throttle.
VTC is Nissan's version of variable cam gear timing, the intake cam gear is advanced a full 21 degrees depending on engine RPM. What this does is increase power and torque at lower RPMs, much nicer to drive. This is not to be confused with VVL which is Nissan's version of Honda's VTEC or Toyota's VVT-i, VVL/VTEC/VVT-i work on changing the lift of valves by changing the cam shaft lobe profiles. The page below is taken from the S15 Service Manual, it explains the VTC function and how it operates, just oil pressure and a solenoid.
There are two main things that causes the rattle, the oil solenoid could be blocked/dirty so you just need to clean that out with some carby cleaner. You can then check if the solenoid is functional by connecting 12V power to it, you should see the pin/piston firing in and out. But the most common problem is just the cam gear wearing out, only way to fix this is to replace it.
When installing Rocker Arm Stoppers in an S15/S14 engine, you need to slightly trim the internals of the rocker cover. I used an angle grinder and then a Dremel to clean it up. Also had to give it a good clean because of all the fine pieces of aluminium, was a pain in the ass.