2004 3.9L v6 mustang knocking and ticking loudly

Okay I bought my 04 3.9L v6  mustang and it ran
like a dream it had 148,000 miles on a recent
rebuilt moter and I noticed that the exhaust
manifold started to slightly leak, but I was
struggling to find the time to get it fixed I drove it
for about three more months my oil was in need of
changing so I Took it to a shop and had the oil
change done. When I was on my way to work the
next day or so later I had stopped by the gas
station to top it off I got back in the car and as
soon as I started it the horrific sound I herd well is
hard to explain almost as if all 4 of the cats where
cut out and that's when I noticed the light ticking &
knocking from the engine on the upper driver side I
noticed that the oil pressure gauge was not
working at all and the check engine light was on
any ideas?