'01 XJ with 132k miles on it. Bought it three years ago and have done a lot of work on it...new suspension, AC system, water pump, thermostat, ignition rail, etc. Last summer I was getting an issue where the oil pressure gauge was showing a drop to 0 while stopped at lights, esp when it was hot out. A few months ago, I got a misfire on #1 & #4. I've recently replaced all the plugs with Champion copper plugs and have replaced all of the injectors. I'm getting a significant clicking/tapping on the engine and what sounds like an air hiss when the engine is running. I've checked all of the hoses going in/out of the throttle and air box. All of the injectors seem fully seated and I'm not getting a gas smell. Oil level is good. 1/2 can of Seafoam added to gas tank and 1/2 to oil fill. No codes are being thrown at the moment. Wondering if this is an issue with lifters and springs? Any other ideas that I can check before taking this in to my local mechanic? Thanks in advance for your help

Here are two videos showing the issue

1 - Engine Bay:
2 - Interior:

 

The tapping sound is definitely valvet-train related. The best thing to do is pull the valve cover and inspect. You will more than likely find a loose rocker arm on one valve. There can be several causes; collapsed lifter, bent push rod, damaged rocker arm or worst-case a bad cam lobe. The engine seems to run smooth despite the noise, so I'd think it should be obvious what the issue is, once you take a look inside.

The hissing sound is probably from the crankcase vent on the back of the valve cover. My XJ hisses the same. Clamp off the vacuum line to it and see if the hiss goes away. The grommet on the valve cover gets hard and doesn't seal very well so you can hear the vacuum hiss from the crankcase vent orifice.

 

This should be in a sticky....

1)if you're really getting 0 oil pressure at idle you have major problems. Put a mechanical gauge on it to see what it actually is at idle and 2000 or

2) with that model year you of course need to determine if a cracked 331 head has dumped coolant into the oil and trashed the bearings.