Possible knocking sound, goes away with 91 octane

My '01 6cyl 4wd manual with 238k on it has developed a knocking sound when attempting to accelerate or climb at lower RPMs (1800-2100rpm or so). It started relatively recently (since June) and when I put a tank of 91 octane in it (normally use 87 or 88) the noise is gone. I drove it across the country in May after giving my mechanic more or less free reign to find and fix whatever he found was needed (radiator, clutch, valve cover gasket, some bushings, et al) but he didn't find or fix anything directly related to how it runs. The only OBD code I'm seeing (occasionally) is something about the evap sensor/purge but that's been an on again off again issue for a couple years now.

Any ideas? Home is in California but I'm in rural Maine at least through October and haven't yet developed a relationship with a mechanic out here. I guess I'd just like to have some ideas in mind before I go looking for one.

Thanks,
Dan

 

Do you have an OBDII reader like a scangauge or an ultragauge? Check your fuel trims. Those will give you an indication of a problem. When was the MAF last cleaned?

 

I’ve been watching this guy for a couple years. Wish he lived near me. South main auto LLC.

https://g.co/kgs/NjaiZK

 

I noticed faint knocking on my 3.4 at about 200k miles, switched to premium. However, I had a weak fuel injector for quite a while and I don't remember if the repair was before that - I think it was. Perhaps I'll try 87 to see if it still knocks but I think many people here are running premium because engine seems to run better and some say better mileage also. Why an engine with only 9.6:1 compression ratio runs better on premium is beyond me but seems to be the case.

 

You might have a cylinder or two that are running lean. A low flowing injector could cause that. Carbon buildup on the pistons would also be a culprit.