I have a '99 Rodeo, 189k-mi, Works fine, but it has a noisy engine shound, sometimes surges when idling. Can't tell what it is. I've tried different Synthetics, Lucas additives, Mystery Oil, etc. And it will not quiet. Timing belt was changed about 2-3 yrs ago. I change oil regularly. Radiator fluid will get changed today, but I'll have it done somewhere. Looks like bong water,
Any ideas what the noise could be? Kind of a whirring sound.