First thing is to check the timing belt if it is a 4 cylinder model. These are notorious for a slapping or clunking noise as the belt gets a bit loose. This is esp true if the belt has been changed as the spring loaded belt tensioner is anstalled at that time(not used from the factory). Once again I cannot emphasize enough that people need to buy a good Manual for their car as it answers most of their questions.
Get a new one.
when stopped at red light or when in park
you have a bad alternater or belt
It turns out that it was a bad idle air control valve. Right after I replaced it the loud noise stopped. About a hundred dollar fix, it was well worth it!
Vacuum hose leaking.
Is the loud noise an intermittent squeak? My 1998 ZX2 had a very annoying loud squeak that turned out to be due to the use of the wrong PCV valve. The ZX2 2.0L engine apparently comes in SOHC and DOHC versions and if you use the PCV valve intended for the SOHC engine in the DOHC engine, you will get the squeak. Before I knew this, more than one garage has installed the wrong one on my ZX2.
Idle air control motor
When you here a rattling noise from the engine at idle.
Sound like your fuel/air ratio is not correct.
idle air control valve
The Ford Explorer uses an idle speed control valve that allows air to flow thru the idle air passage inside the intake manifold. When this air passage gets clogged up with carbon or grime it restricts the passage & the result is a loud humming / moaning noise. The repair is to remove idle speed control valve & spray carburetor cleaner or intake cleaner thru air passages inside intake manifold & clean or replace idle speed control valve.
Its a loud Knocking sound. sounds like some one is in your engine with a hammer just beating the heck out of the inside of the motor. The Knock is slow at idle then speeds up when the RPMs increase. means Crankshaft bearings are bad and crankshaft is ka put. David G
1998 rav4 engine wont idle and clacking noise from under timing belt cover
Added info to question: Vacuum also coincidently went from steady 20psi to 15psi/knock - small backfire/miss on acceleration from idle. Knock is steady at idle.
I have a 1997 Chevy Malibu with 176k on it. Never had a new engine or anything, runs like a champ. It has been making a knocking noise when its cold.(until car is warm) for a long time now. It still makes a little one while at idle, not loud or can even really be heard though.
Have you checked your Exhaust Muffler and pipes ? Probably the fuel pump
Have valves adjusted.
Extra loud engine sounds. High idle speed. Lean burn condition that could result in engine damage.
Possibly just an adjustment on the rockers. Hydraulic system. Hopefully that's it.
The idle control valve works by regulating the idle speed of the engine. The idle speed of the engine is usually controlled by the engine of the computer.
The fan blade may be loose. You can determine this when the engine is off by moving the fan blade by hand or by watching it rotate when the engine is running. if its a real loud then i would check out the harmonic balancer. to check this. with the engine OFF grab the belt and try and move it back and forth if it does and u hear a noise than its the balancer. i had the same problem i fixed it my self saving $$$$$$$, but it was a pain....u need atleast 2 people to fix this. good luck On my 94 Buick Century the water pumb made a loud clanking noise at idle, which would go away when revved up. A day later the water pump seized up and the belt smoked up a storm where it rubbed over the stuck pulley.
I had a very loud humming noise coming from my 99' cougar. I talked to many mechanics and nobody knew what it was. I looked it up online and found two sources( listed at bottom) that said it was the idle air control valve. I replaced the part and my car runs great without the horrible loud noise.http://wiki.answers.com/Q/1999_mercury_cougar_-what_is_the_fog_horn_like_sound_mean_after_it_is_started>;