Loud knocking like a rod

I have a 2003 Ford Mustang convertible with the
V6 and 3.8 engine it about a week ago I got it up at
9 on the highway and it started to not I completely
shut it down slower and it went away however
when I started driving the next day I can hear
knocking I had mechanic listen he said I thought I
might have a lifter maybe a main bearing check the
oil it was a little bit low but I continue to drive it
only has a hunt 81000 miles on it having the
knocking that so loud yesterday that I parked it
because I was afraid I was going to throw a rod or
blow up my engine and I'm just curious as to what
it could be and what it might cost to repair