2004 Toyota Matrix engine rattle

It sounds like you are getting engine ping. If a higher octane gas cures the condition, then have the base ignition timing and timing advance checked. It is also possible that you are shifting gears too soon and lugging the engine. Does shifting to a lower gear make the problem go away?

Are you sure it's really engine noise? I hear the same niose in my Matrix, but it doesn't go away when I shift into neutral and let the engine idle down. I only hear it at highway speeds.

I'm pretty sure that what I'm hearing are the plastic wind panels below the engine rattling because the plastic fasteners are loose.

If it IS 'spark knock or pinging' (detonation) then you'll only hear it when the engine is 'pulling hard', (and especially at high RPM)...let off on the accelerator and it'll stop.

Wouldn't that be high load/low rpm? If you let off the accelerator OR downshift to increase the rpms it will usually stop.

That was the point I was trying to make--I hear it regardless of what the engine is doing. That's what makes me think my particular noise isn't coming from the engine.

Now the noise I'm hearing from my wife's old Corolla on the other hand.... I think that's piston slap that happens until it warms up and the pistons/rings expand. The little fellas have just gone up and down in those holes so many times they don't fit quite like they used to.

The OP said his noise was more noticable at low speeds, which I missed on the first reading-- that doesn't sound like road/wind noise. I think your comment about if it's happening under load is a good one to help him diagnose what's probably going on.

Well, generally if you downshift (so that you have higher RPM) then your power will increase and you'll let off on the accelerator some, this'll reduce the pressure in the cyl and it'll stop detonating...

waited until I was down 1/2 tank regular octance gas (87), toppped off with next level higher (89) and it seems to run and sound much better. When this tank runs out I'll refill with regular octane (87) to see if I get the noise again. If so, I'll have the dealer check the timing. Thx