Might have spun a bearing....
 

Well, I'll try to keep it short. Took the turbo off,engine wise the car is back to stock. It was sitting for about three weeks undriven while I worked on it. Took it for an inspection the day everything was finished and it passed. Everything seems fine, and then later that day I'm cruising at about 30mph and out of no where a horrible knock sound starts. Like people turn to look as I drive by it's that loud. Pulled over, saw nothing hanging or banging on anything kept driving and the sound suddenly became very quiet, barely audible over the exhaust but still there. Drove it again today and it was again very quiet except for about 2 minutes when it became very loud again before quieting down just like before. Also oil was not very low, only less then a quart low, and nothing funky with the oil pressure.:vash:

Things I've tried:

As per this thread Spun Rod Bearing - MX-5 Miata Forum I tried pulling each plug wire individually and heard no difference, the sound did not disappear.

Ran an oil flush through the engine and changed the oil today hoping it was a cranky HLA. It had probably been ~3500-5000 miles since the last change. No difference in the sound since and no metal in the oil as far as I could tell.

So, could the A/C, water pump, oil pump, or alternator make a sound like this? How about timing belt tensioner? A totally collapsed HLA? Or is a bearing toast?

Here's a video I made tonight. I'm standing in front of the engine bay revving it by hand.

Would you say it is the same as crank speed, or half of crank speed?

Half = something in the head, like a collapsed HLA,
Same = Maybe a bearing.

-Rouphis

Sounds just like my stock engine that had a spun/hammered bearing :(

But it is a youtube video so don't take my word on it 100%.

Its always a good excuse to get a built motor though :)

edit: yes what rouphis said, does it increase with engine speed or worsen with engine load?

It definitely increases with engine speed. I don't know about load because I've been extremely gentle with it since the noise began. How would I tell if it's half crank speed or at crank speed?

And no built engines. Financially this could not have come at a worse time for me. I'll probably have to sell the car if it is a bearing.

When I was changing stuff out on my Integra, I didn't push the spark plug boot all the way down and it made a knocking sound.

Don't wanna spoil your day but about a year ago my friends car sounded like that and a rod went through the oil pan after a two days of knocking noise

Any particular reason you would have to sell and couldn't just let it sit until you could do a little bit of a rebuild? Is it your only car?

find a way to make it work, don't sell it.

yeah jhon i wouldnt sell it if i could help it. Was that video at idle? or driving around i would take a stick and use it like a stethescope and listen to the valve cover etc until you find the area where the sound originates then investigate further from there on. It could be a hla, or something much worse loose wristping spun bearing etc you will not know until you investigate it thuroughly. If nothing else and the sound is from the head take off your valve cover and check your hla's if you pull that puppy off and there is one sitting down in its hole away from the cam i'll give you 2 gueses as to what is wrong gl man and keep us posted.

That does sound like half crank speed to me, so hopefully its something in the valvetrain and not a rod bearing. I wish you well my friend. The last thing we need is another person with a blown engine.

Then again, wouldn't you likely hear the knocking more on combustion stroke rather than combustion and intake strokes? So I guess it could only be half crank speed that way. At 1000rpm crank speed would be around 16 rotations per second. Half of that is of course 8 for those not handy with math. That sounds about like 8 knocks per second to me, rather than 16.

Dont know how hot it is in NYC right now but you could try to run a thicker oil, maybe that'd help if its a valvetrain noise.

I went from a 5w30 to a 15w50 and it help the valvetrain chatter I was getting, for awhile until the bottom end of the engine completely fucked up but thats another story lol.

I wouldn't sale it either, although I've thought about it myself. So far in now, why turn back? If it is a bearing and manage to have it sit for awhile, cheapest route could be just pulling the engine tearing it down, getting oversized bearings and then be out some assembly charges. Unless you can handle it yourself!

Best of luck to ya, I feel your pain lol.

Given the 1000rpm/16 rps, that would be 4 combustion strokes a second..so 4 knocks per second...2 if it's at half speed.

I've had a intermittant issue where when I start my car when it's completely cold, it would make a rattle/knock-ish noise. I would immidately turn it off (given hearing it, reacing to it and turning the engine off..it would make 2-3 seconds of noise) and re-start the car....upon re-starting the car it would cease to exist :dunno: Only happened right after a cold start...and one time I let it keep going and it went away after a few seconds. Hasn't happened in quite sometime though...maybe it was bad oil at the time.

Oops, very true that would be 4 knocks per second. Its surely more than that. I guess I go back to it being valvetrain.

That's a good idea, thanks, I'll try it this weekend. The vid was at idle revving the throttle with my hand. The only reason I'd sell is because I can take the train to work and I am literally dead broke so I can't swap in a new engine if that is the issue, and I have no where to store the car for an extended period of time. I live in the city and it's a trek to go to my parents place in the burbs to work on it on the weekend. Cash in my bank account is more valuable than a steal brick at the moment. I'll investigate further this weekend and update this. Thanks for the suggestions so far.

Sorry man, sounds just like how mine did although just a bit quieter. My bearings were all jacked, and #2 rod bearing was spinning.

Time for rebuild.

This happens to me all the time.

Drain the oil and look for the terrible brass sparkle in it.

Yep. This happened to me on my pretty much stock 2000. Nothing like rod knock to ruin your week.

Well, believe it or not the sound has disappeared. I guess it was a stuck HLA. I just never imagined a lifter could be that loud. Either way, as long as no new surprises show up I'm a happy guy.

A good diagnostic tool for this kind of thing is a piece of hose or a screwdriver used as a stethoscope. I have diagnosed many engine noises with a long screwdriver, put the tip of it where you think the noise is coming from, and put your ear to the handle.. you can find 90% of noises that way.

now that it has stopped, it is the time to sell the car :)