Exhaust manifold noise

My 89 4.0L Cherokee w/108k miles has begun knocking/rattling from the exhaust manifold. It seems to me that this bagan when I upgraded to 91 octane fuel. Could the increase in octane be responsible for this noise. Any info is appreciated. Thanks.

Why did you upgrade to 91, kind of a waste of money... If it was knocking BEFORE the upgrade you might have replaced the knock sensor first.

I switched because a friend of mine told me that when he switched he had a noticable improvement in gas milage and a smoother idle. As far as I can remember the knock was not present before the fuel switch. I did a tune-up yesterday (plugs, wires, cap, rotor) and that helped a little with my slightly rough idle, but I'm now thinking that maybe I have an oxygen/knock sensor problem. Could the higher octane gas have damaged my knock sensor and/or oxygen sensor?

Not really, most XJ's suffer gas milage wise if upping octane for no good reason. The 4.0 is a low rpm grunt motor not a 4 cyl ricer. All higher octane does is prevent pre-ignition on high compression motors and thats about it. Oh, some might have a bit higher cleaning stuff, maybe, or so they claim.

Yeah, I did the higher-octane experiment a few yers ago while driving 130 miles a day. The only noticeable change was that my wallet got lighter, faster...