My 86 Chevy C-10 knocks and pings when I step on the gas when it is in gear. When idling it sounds perfect, could it be a vacuum leak?

There are basically two types of whistle noises.

* Whistle when you get more gas (increase RPM, put gas pedal, sometimes just idling but louder when you put more gas)
This usually intake-side leak.
Intake manifold itself, intake manifold gasket (most likely), even airfilter housing, Airflow meter housing (connector), anything where air goes to engine.
usually engine get rough or unstable idle when intake leak occur.
Engine knocking or pinging may feel.

* Whistle when you decrease RPM (engine brake, reduce speed, sometimes just idling but louder when it dropping RPM)
This usually Vacuum-side leak.
There are many vacuum hoses in engine room. even inside of cabin (heater control, etc)
Try to spot where it coming from.
Vacuum also connected to Brake servo, heater control and spark timing.

Put metal or plastic pipe (small PVC pipe will work fine) to one of your ear then isolate source of noise and find where it come from.

these trucks are equipped with an idle speed control motor. What you're describing sounds like that part has stopped working. The part is designed to maintain idle when gas pedal is released. Now if you hear an audible hissing noise under the hood, then theres a vacuum leak which can cause the same problem, but it sounds like the idle control motor. Good luck.

Many older Ford products don't have anti knock sensor circuit so most likely your hearing your cylinder valve's death rattle. Higher OCTANE gas will help. Break a valve junk the car.

check the vacuum lines going to the tranny, but it sounds like a vacuum leak, when the engine is at idle, spray a few short blasts of starting fluid around all the vacuum lines, if the rpms jump up, thats where the leak is

A bad EGR can make it ping..Also a low Barometer sensor can also make it ping...But first i would check the knock sensor

Hello, Thomas. Check the knock sensor. This sensor is suppose to send signal to the computer if the engine start to knocking or pinging. The computer will change the setting on various other components to keep from doing this knocking or pinging. I remember in the older cars that weren't fuel injected but carbureted and had to burn high octane fuel. If one put low octane in the fuel tank the engine would knock/ping, and continue to run after the ignition was turned off. Try the knock sensor or do an diagnosis test on it. Look in your owner manual it tell how to self diagnosis test on you car. GB....stewbison

Sounds like a vacuum leak. this will affect the idle causing it to idle high but with no power (at idle) so almost dies at idle in drive, (under resistance) The knocking is likely your exhaust pipe shaking and hitting under car, cos of the engine shake.
Look for a vacuum tube hanging loose from around the intake area. Maybe trim off stretched end and reattach. May be able to hear it with engine running.

Usually the rotten egg smell is a clogged CAT. Is the engine light on and if so did you retrieve any codes?

The fuel pump should be in the 20s. Also it could be losing presure slowly. Leave the fuel pump pressure gage on for at least 2 to 3 hours to see if it is losing pressure - It's a start

you will distroy the engine if you continue to let this noise happen.
the timing must be retarded to get the ping out if its that bad.
most of the time if its just a little bit though filling the tank with super unleaded will take out the noise.
the distributer must be loosend and turned in the correct way to ****** the timing and the idle must be put back up to compensate.
the distributer is the ignition timing.
the other thing that could have happend is the timing may have jumped time on the timing belt itself.this is a complely different timing.
this timing is valve timing.
the distributer is ignition timing.
when the distributer is turned you are changing the ignition timing .
you say in your text and i quote .

The service tech only adjusted the distributor fas far as he could to lower the idle.

a tech should never be turning the distributer to lower the idle nor should this be explained in any way to the customer.

if a tech says he dosent understand something stop them rightaway and get your car out of there or get someone else to talk to .
10 times out of 10 if the tech dosnet get it the car wont get it either.
have a competent service tech test and conferm the timing belt or chain timing .

keep me posted
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thank you
jm