Metallic 'knocking' Noise

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Hi all, my 97 Corolla SD, auto 1.6L, makes a metallic knocking noise during idle but only after warmed up, and again while driving but only coasting, ie. not during acceleration. Any ideas? Thanks in advance.

Could be a number of things causing the engine to make a metallic knocking noise after it warms up. Need a bit more information:

Does the knocking/tapping noise seem to follow engine RPMs or does it follow road speed?

Anything that seems to make the noise worse, ie. slowing down to a stop, after running on the highway, after running primarily in the city?

Did this suddenly start or happened over time?

Did this happen to coincide with any other repair event, right after an oil change, right after timing belt service, etc?

Couple of things to check on (basically the maintenance items):

- pull the sparkplugs to make sure they are still in good shape

- check cap and rotor for carbon tracking / excessive wear

- visually check the conditions of the plug wires, check resistance of them if you have a multimeter - do them one at a time to avoid messing up the firing order

- visually check that your vacuum hoses are in good shape, check for dry rot, hardening, cracks, etc.

- check all fluid levels

- check condition of air filter / give the throttlebody a good cleaning helps as well

My feeling is that this is related to engine vacuum / EGR operation. Since it happens when the engine is warmed up at idle, and again when you are coasting - those happen to coincide with maximum engine vacuum generation. Could be a timing issue (too much advance, detonation/pinging, piston slap), a vacuum leak (makes for poor idle, affects EVAP operation), clogged/stuck or faulty EGR valve/vacuum modulator (most likely for this generation) that is not recirculating enough exhaust gas to cool the combustion chamber mix. EGR valve can be cleaned out, but it is hard to get to, due to its location. Vacuum modulator is ontop, right by the plenum for the intake manifold - as well as the vacuum connections, you should be able to visually check the hoses there. Vacuum modulator can be pulled apart to verify the flapper valve is still functional.

Another possibility is valvetrain noise and/or damage bearings - though those tend to make a very distinctive noise and be there on/off the throttle.

Thanks very much fishexpo101.

In answer to your questions, the noise doesn't seem to follow acceleration, or braking for that matter. Seems to happen only on idle and coasting. No specific driving action seems to make the noise worse, otherwise. It started making this noise suddenly, not over time, and not after any repair visit or service. Haven't had service in months, probably due for a tune up. The vehicle has close to 170k miles.

Thanks for the tips, I will check all those items and post back. Maybe it is something to do with the vacuum after all.

Last question, does anybody have an electronic version of a repair guide for this vehicle?


Autozone main website has a link to an electronic repair guide library that I believe covers the 7th gen Corolla, don't remember seeing an 8th gen Corolla, but I have been on there for some time. Lots of the documentation there is nearly identical to a Haynes manual. Good place to start to gather information, then you can decide if you want to buy a repair manual yourself to have handy.

Thanks Fish. I think I have bigger problems. Popping the hood, I noticed it's almost certainly engine noise, a constant noise that accelerates in frequency when given gas. This is reminiscent of the noise my Volvo S60 made when its engine died in December.

Also, it loses power, almost as if a cylinder is dead. Gonna (slowly) drive it in to the local Crappy Tire (closest mechanic to my house) and hope for the best.

Ouch! Hopefully it is something like a dead plug (completely fouled) or a bad spark plug wire. The 4AFE are pretty tough little engines. Let us know what it turns out to be.

Thanks Fish, but I took it in, the engine is damaged. I need a new engine, $3k, including labour and tax. Does anybody know if I can get an engine cheaper than this, say less than $1k? Ridiculous to pay so much for such an old car.

The mechanic is quoting a $900 engine plus tons of other stuff, including 16(!) hours of labour, which I find ridiculous. I'm looking to get a $500 engine shipped to me from Toronto (I'm in Ottawa) and installed by somebody at much more reasonable labour rates.

Wow, $3k for an engine!!! Even with installation + tax, way too high for a 4AFE. I could understand dropping in a different generation of engine, needing all that fabrication, rewiring, etc. - but for a drop in swap, can't see it costing that much.

You can get a reman long block (nearly complete engine) 4AFE with a 12 month warranty + core is around $1700. Good used ones can be had for around $350-$850 - depending on your area. Also depends on what was "damaged" - the 4AFE is pretty tough, damage that I can see would be wiped bearings or warped components. Could get a rebuild for considerably less than an engine swap.

If you do not have access to a salvage yard with applicable donor cars, you can try EBay, Craigslist, or other local sources. Might be able to find a clean salvaged car (insurance total-loss vehicle), just grab the bits you need and part out the rest to recoup some of that money.

Only sticking point would be model years available for the 4AFE, some jurisdictions will not allow you to swap in a 1993 4AFE into your 1997 car, even though the engines are nearly identical - due to emissions regulations. Doesn't mean that it is impossible, indeed, you can keep all the existing pollution control systems in place and just swap in a head, block, parts that are needed to repair your car.

Curious - did the mechanic elaborate to as what was "damaged"? Might be something that could be fixed for a lot less.

I'm as shocked as you are, Fish. He is quoting $950 for the engine itself, then a couple hundred for some parts, and fluids, coolant flush (is this needed?), etc. The thing that I find ridiculous is the 16 units (hrs) labour quote, which even at $75/hr, greatly inflates the quote.

He hasn't told me what is damaged, I will get that info before driving away. Meantime, I am looking for labour quotes from other mechanics (I only took it to this Crappy Tire outlet because of proximity to where I live, but now I am OK with driving cross-city to save some money).He was pretty clear it wasn't reparable, ie. needed engine replacement.

OK, I found a cheaper used engine that I can get for $500, I just have to determine if it can 'legally' be installed on my 97 (if it isn't a 97), as per your emissions comment, thanks for that Fish, it never occurred to me. And installed in fewer hours.

I'm not ready to say goodbye to my beloved 'Rolla just yet.