2.5L engine noise/rattle

[verTego]

Dear all,

I have read all the engine noise articles I can find, but none really match the sound that my engine is making.

The noise appears from start-up, but is quite feint until I rev to 2500 rpm at which point it gets really noticeable (more or less disappears after this rpm again). It sounds like a sort of rattle, a little like a bunch of marbles gently shaking around in the engine. Its slightly muffled so I don't think its an external rattle (bolt missing, exhaust etc). The noise is unaffected if the engine is loaded or unloaded. I tried switching A/C on and off, no difference. My free time is a little constrained and space/tools are not available so diagnosing with the car lifted/use of stethoscope etc is difficult.

Any simple-ish diagnostics you could suggest would be great before I "bite-the-bullet" and hand her in! Thanks in advance for your time!

Car is a 1998 2.5 manual shift with 116,000 kms on the clock.