Help - 2001 Blazer knocks while accelerating or going uphill

I have a 2001 S-10 Blazer with about 60k miles.  It recently started
knocking while going uphill or while accelerating.  It almost sounds
to me like a diesel engine.  Otherwise it runs fine. Its seems to do
it  a little worse just after the first start of the day.  It may be
coming from under the hood but it almost sounds like its some exhaust
thing.  I tried using a tank of 93 octane gas instead of 87 and that
didn’t seem to help (only maybe a tiny bit).  

Anyway, I’d prefer not to spend  a bunch of money if its something I
can do.  Does anyone have any suggestions where I can start looking.

Thanks.
PS

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