Mach 1 - strange engine rattle

[rovorster]

Hey all. I have a 2003 Mach 1 with about 130k miles on the clock, use it mostly for spirited weekend driving and commute about 2 or 3 times a week. Since ownership have changed the oil every ~6 months or so (probably around 5k miles) and have had no issues with the car. Recently it started to make a rattling noise and I can't quite place it, these are the symptoms:
- Sounds like a metal rattle, reminds me of the cold start lifter tick my Miata used to have.
- Most noticeable around 2.5k RPM, not there at lower RPM's but at higher RPM's it's still there
- It comes in once the engine starts to warm up (~2-3 mins into driving)
- It's there only under light load, if I'm off throttle or giving it just a bit more gas (like over a quarter throttle or so) it disappears completely. It'll consistently be there under e.g. light throttle on the highway when going slightly downhill. If I freely rev the engine you can hear it very shortly when the revs are on the way down.

Revving it under the hood, it sounds like it's coming from the back of the passenger head underneath the hood, but it's pretty hard to pin down. I tried firmly holding the shaker and that didn't seem to make a difference. The car is definitely not down on power and feels healthy. Only problem I noticed recently was when I changed the spark plugs a while ago; one of them seemed to have a bit of oil on the plug wire, figured it was just one of the seals and not a major deal. From some initial research seems like it could be:
- A rod bearing (very worried about this one because I'm afraid I'll need a new crank)
- Some kind of valve lash/lifter issue. But then I'd really expect it to show up specifically when the car is cold, and it's the opposite of that.
- Timing chain tensioner? Again would really expect to hear this only when the car is cold.
- Clutch in/out makes no difference, definitely sounds like engine internal to me.

Anyone have any other ideas or heard a similar noise? I love this car and am willing to put in the money to keep it running. Plan is take the timing chain cover off over the weekend to look at the tensioner + take the passenger-side valve cover off and hopefully find something.

 

[P49Y-CY]

i'll throw this out there since something similar happened to me and it was one of the last things i was expecting. do u still have the stock midpipe?

last year i put heads on my f150 4.6 and all was good for a couple of months until i started hearing a noise with very similar conditions. could have sworn it was a rod knock but went away under wot.

turned out that the substrate material in one of the catalytic converters was starting to come loose.

best of luck with it.

 

[FleeMan]

+1 as this happened on my Suburban as well with the factory midpipe. Cats were starting to fall apart.

 

[DSG2003Mach1]

was it rattling before the plug change as well? An exhaust leak/rattling can also be a sign of a plug ready for lift off because its backing out.

 

[rovorster]

Hm, checking the cats is a good idea, it doesn't have the stock midpipe though so it seems unlikely, but definitely worth checking.

@DSG2003Mach1, yup it was rattling before the plug change, I was worried about blowing a plug (seems to be a pretty common Ford V8 theme...). Didn't seem to make a noticeable difference to the rattling noise at all.

 

[01yellercobra]

I blew the cats out on my Bassani mid pipe. I've seen more issues with aftermarket cats than stockers.

That kind of sounds like how the main tensioner acted in my 01 when it started going out. I could hear it at part throttle and idle, but not at WOT. Of course at WOT I couldn't hear much of anything on that car. But I could definitely tell it was coming from the front of the engine.

You can check the rod bearing by pulling the COP for each cylinder. I haven't done this myself, but supposedly without the pressure on the piston it'll get quiet(er). I don't know of any way to foolproof check the lash adjusters. So maybe just plan on replacing those when you replace the seals.