2000 Silverado Hiss Noise

Year 2000 6.0L engine. A loud hiss noise like pressure releasing under the hood which repeats every 6-10 seconds when driving. Doesn't happen at idle so I can't stand there and listen. Ideas? I suspect a/c related.

Year 2000 6.0L engine. A loud hiss noise like pressure releasing under the hood which repeats every 6-10 seconds when driving. Doesn't happen at idle so I can't stand there and listen. Ideas? I suspect a/c related.

check the exhaust manifold bolts to the engine for failure.

Thanks, will check later

I had a similar sound from my 02 Silvy, but usually occurred at highway speed.
After about a month, it got noticeably worse, turned out to be the idler pulley, bearings were completely gone, (I mean, no longer there!). The grooved outer plastic ring was there, with belt still running true on it. The outer bearing race was spinning on the stud, all balls and iner race were gone. I had just completed a 500 mile highway drive at 80-85 mph, I was amazed at what I saw.

Yes and No.
Yes, when I check the exhaust manifold bolts the heads of the front and back one are missing on the passenger side and the back one on the driver's side; but I have to say I don't notice any signs or sounds of an exhaust leak.
Today I just picked up a load of mulch for the yard from about 5 miles away. No sound on the outward journey until I kicked on the a/c on the way back and.......psssssttt..........psssssttt and stop when I turn the compressor off.
St Pat's Day and this buoy from the County Down knows rather little about the a/c system!

have all the bolts replaced and new gaskets on the exhaust manifold.

this is causing the MPG to drop because the knock sensors will retard timing with this issue.

It looks like I'll need to pull the heads to get those back bolts so on a vehicle that normally sits at the side of the house it's not a priority right now. I'll want to get the a/c sorted though for the weekly trash and various other short DIY pickup runs as the weather warms up.
It sounds like a pressure relief valve under the hood going off about every 10 seconds and with the window down it's loud! Anyone any ideas?

My wife's 2000 Silverado with a V8, started doing the same noise. It is the AC, but I have not checked into the noise because it just started yesterday on the way to the doctor's office. It did it all the way to the office, but on the way back it was fine. I will have to get the gauges out and look to see what is happening to the AC pressures. Low side and on the high side.