Few thoughts from your initial diagnosis. Most common sssss noise would be a cracked air intake hose that breaks from wear and tear (plplplpl can provide the example pics) [you're right about vacuum leak if it's this]. Take off hose from airbox to throttle body and shine with flashlight to see if any wear or cracking is present.Hi all,
Starting last week, I have a constant hissing noise on acceleration in my pilot. It doesn't change pitch or volume based on RPM, just the same droning ssssssss noise whenever I have my foot on the gas. If I let off the throttle it stops instantly.
It seems like I have a little less power, definitely slower getting up to speed, and my gas mileage is WAY worse. Seems like it still shifts and handles fine otherwise.
I thought maybe a vacuum leak, but didn't find anything with an exploratory can of starter fluid.
Second possibility could be exhaust leak near flex pipe or catalytic converter. A simple inspection under the vehicle when it is cold could help find the leak if any. Maybe even a rust hole in the exhaust piping or muffler.
Other less common but still possible culprits could be: worn out throttle body gasket or open vacuum line somewhere (brake booster?). If you have a fairly advanced scan tool that has datalogging (or rent from O'Reillys or Autozone)- you could check what the air to fuel mixture is, along with the short term and long term fuel trims which could help narrow down if you've got a vacuum leak or cracked hose somewhere.
You can also make a $20 smoke vacuum leak tester easily that will allow you to check for vacuum leaks- just need a mason jar with lid, soldering iron, some thin plastic hose, hand pump (ideally fluid pump with input and output), rag / shop towel, and baby oil. Here's how- Grab soldering iron and mason jar w/ lid. Make two holes in lid one for soldering iron to sit inside pointing down and one for the hose that will connect to hand pump- make holes tight so smoke doesn't escape. Put in rag and pour in baby oil so most of soldering iron tip is submerged. Plug in soldering iron. Route hose in lid to input side of hand/ fluid transfer pump and have second hose ready for making smoke; keep hose as high up as possible and out of the oil- you just want the smoke. Wait for soldering iron to heat up for a few minutes and you'll have a thin cloud of smoke inside mason jar and you can begin putting the output hose into where you want to check for vacuum leaks and slowly pump the hand pump to transfer the smoke into where you're looking.
This trick saved me a few hundred dollars by finding a tiny cracked vacuum hose that I was able to replace for 3 dollars. A cheap shop wanted $250 just to run diagnostics with their smoke machine. Just a cheap way to find out if you have a vacuum leak if it's not the other possibilities.