1994 6.5 turbo,excessive noise/rattle
 

I have a 1994 chev ex-cab 1500 2wd with 4l80e automatic trans.truck bought new by my dad,the only work ever done to it was, injection pump replaced and remote fuel filter installed appx.12yrs ago.truck has 276 thousand miles.idont drive it much.it has a loud rattle noise seems to be coming from the frt of the engine?(seems to be louder when idle quiets some when under load.its been doing this for a while and has not blown apart yet.all accessories on frt have been replaced thinking bearing trouble(new alt,a/c comp,vac pump)
could it be loose timing chain,valves or valve timing,cracked flywheel.would like to keep truck going.it has a lot of sentimental value.truck also blows lots black smoke under load and gets poor fuel mileage.any ideas thanks alot.