Vacuum Check - Possible leak?

[quatin]

I just noticed a "sucking air" sound from the intake manifold area. I attached a vacuum pump with a gauge to the PCV intake hose and found the needle jumping between 18-22 at idle. When I accelerate to 2k rpm, the jumping gets worse and when I release the gas, it jumps to a steady 25 and then returns to jumping between 18-22.

This site suggest weak valve springs, which I can't rule out, but what is the sucking sound?
Technical Articles: Engine testing with a Vacuum Gauge - at Greg's Engine & Machine

I recorded video of the events.

Sucking air noise.


Vacuum check.

 

[SilverRidgeRunner1]

Smoke that intake will find u every leaking vacuum hose on the engine.

 

[quatin]

Paid $50 at pep boys for a smoke test. The tech was kind enough to let me watch. We found a small leak from a loose intake hose clamp. There was also a lot of smoke from the EGR valve/modulator area, but he said that's normal. He tightened the hose clamp and we checked the vacuum with his gauge which read a steady 20.

Problem is, I still hear the hissing sound. When I got home, I attached my vacuum gauge and I still see the gauge bounce rapidly. I'm confused why my gauge shows it and his did not. When I give a quick tap on the gas pedal, the pressure drops to 10. When I slowly accelerate, it starts swinging between 15-20.

It's hard to get guys to work on the car when there's no apparent problem. They looked at me weird when I said there's no CEL and no real performance problems, but my vacuum readings are off. I guess it now looks like either an exhaust block (had my cat stolen and have a cheap cat on it now) or it's weak valve springs.

 

[SilverRidgeRunner1]

Or that same clamp worked itself loose again if the noise came back. Did u recheck when u got home... Vacuum leaks are hit or miss on turning a Chk eng light

 

[quatin]

I took the car to my buddys shop this morning. The tech and I spent an hour going over it. He got misled when he couldn't get a vacuum in the fuel pressure sensor and thought the EGR check valve was bad. I come home and find out that 3rd gens don't run a vacuum to the fuel pressure sensor.

Meanwhile, I replaced the PCV valve, cleaned the throttle body and MAF sensor. I noticed a lot of carbon deposits, so I ran some Seafoam through the vacuum lines. I took a drive to burn the Seafoam off and could immediately tell that the acceleration was smoother. I get back, attach the vacuum gauge and the pressure drop is gone. The needle still bounces, which is most likely weak valve springs, but it no longer drops to 15 inches during a slow acceleration.

I'm happy and confused. If the air intake to the cylinder is restricted (Maybe due to oil leaks from bad valve timing from the springs), wouldn't I get pressure spikes? Why would I lose vacuum pressure during slow acceleration?

Also, the tech said one of my O2 sensors is just not getting a reading at all. They're both 2 year old Denso sensors. However, I do have a cheap aftermarket CAT since my original was stolen. Could the seafoam resolve this issue as well?