Odd knocking noise
 

My 90 has had this odd knocking noise that I cannot figure out. From inside the car it sounds like a hammer is hitting the bottom of the car when the RPMs are below ~2000 and there is no load on the engine (noise slightly varies with RPM). If the car is ever over 2000k RPMs or are starting to accelerate from idle the noise goes away. Outside the car the noise is very faint to non-existent.

Here is what I have checked:
Exhaust - no rattle when shaking it violently
Motor mounts look to be okay, no excessive play
Engine runs smooth, doesn't dirty oil quickly either (one thought was spun bearing, but car has kept running well while trying to diagnose this)
Interior is completely clear of junk, nothing is rattling there
Nothing looks to be hitting against the chassis from the engine bay or underneath the car (still might be possible though)
The oil pressure gauge sits on the 3 of the 30 at idle (where it always has since I have had the car)

Any ideas as to what this could be?

Is it the tick from the hydraulic lifters that every 1.6 makes?

No... the car has slight lifter noise, bu this noise is noticeably different. This is a deeper noise that seemingly can be heard easier from the interior than the engine bay.

Check to make sure the lower alternator bolt hasn't backed out against the intake manifold brace.

My car makes the EXACT same noise.

When at idle it sounds like someone is knocking on the passenger footwell / floor area. Or maybe more the middle / trans tunnel.

Can't hear anything outside of the car, and when the top is down it's harder to hear. I have not figured it out. At first i thought it was spark knock but after tuning, and leaving it this way for a year it would have "solved" its self if that was it.
My other theory was exhaust leak, but again after a year there are no signs of a leak. Nothing hits. Im doing an engine build here soon, we'll see if it goes away after that.

I think it's just engine / exhaust noise echoing around in the car or something. I have no clue though.

Mine does the same with the top down as well. I have driven the car for thousands miles with this noise and just can't figure it out. I would assume if it was an engine problem it would have made itself noticed by now (I don't care if I blow up this motor as I have a 1.8 waiting to go in when the 1.6 gives way).

I'll check the lower alternator bolt, but I am pretty sure that is tight since I changed the belt just a few months ago and it seemed fine then.

PPF bolts tight?

Last time I checked they were.

Has a slight misalignment of the PPF frame ever caused a rattle/knock for anybody?

Another thought:

Does the noise go away when you depress the clutch, or even if you put light pressure on the clutch pedal? At rest, there's a bit of play between the actuator arm and slave piston end. If it's dry (there should be a glob of moly grease there), it will make a rhythmic tapping/clunking sound.

Mine doesn't.
But i also don't have a "tap" i have a "knock" / deadened knuckles on trans tunnel/firewall/floorboard.

I'll start it this weekend, and see if i can capture it on video.

Noise is there clutch in or clutch out. I have grease on the slave (replaced that this winter). I'll also see if I can get video this weekend as well... hopefully my camera can catch the noise.

Erat, u sure the downpipe isn't hitting the tunnel on accel?

It only happens at idle. Or if i free rev it to about 2000rpm like OP said.

I didn't think it was this fast... But either way i don't know.

Maybe it's just lifter tick being resonated into the car... I haven't got a clue. I figure if it is some type of engine knock it would have made itself more apparent by now.


I should also add that it only does it once warm.
We'll see if anything changes after my engine build.

Sounds like something loose up under the dash.

One possibility is the various parts of the HVAC system. They're held together by a type of large, overcenter strap. Foam was used to eliminate rattles. Foam doesn't last 20 years.

Anyway, recommend you get your hand up there and press on things to see if it goes away. Could be almost anything.

That would drive me crazy.

I dont agree about it being something loose and rattling; its too consistent. My first thought was maybe an exhaust leak, but you'd expect it to be louder on the driver's side of the car. Might it be one of the solenoids that live over on the passenger side of the car, or even the EGR?

An old timer mechanic taught me a cool trick using a long screwdriver or pry-bar with a handle as a stethoscope.. Put the handle against your ear and touch the tip to various things under the hood to see where the noise is strongest. Works great.

I can't hear it under the hood at all. Not one bit. Top off, i can hardly hear it. I think it's still there, but is harder to hear. I have no EGR.

I swear it's not an engine related issue. But, like i said. We'll see after my build. I think it's something rattling, it does run a bit rough at idle.

Under the hood there's a bunch of other noise that would mask it, which is where the Screwdriver-Stethoscope method really helps. If one of those solenoids on the passenger side wheel-well is rapping it it might be audible in the footwell; it's only like a foot away from the firewall, y'know? Give it a shot, it's simple and only takes a minute.

Resonance. Also explains why it goes away when the RPM increases.

It'll likely be a pain to find. Good luck.

BTW, I've got one of those on the driver's side. I found it, but haven't fixed it yet. It's WAY up there.

I recommend that you drive fast and spend minimum time at idle. :dealwithit:

I like this idea!