Bought my truck in jan. w/ 90,000 miles. I started using fuel additives & noticed a solid 2 mpg improvement (w/ lucas). I also seemed to hear more fuel knock or valve clatter. It got worse on one particular tank of fuel (running howes m.k.) & then went away completely. It was very loud even at idle but didnt last long enough for me to do anything about it. I noticed my mpg go from 17.5 the tank before (average 16+ usually, even w/o additive) to 14-15, which is about where it was when I bought it. Also it seems my overhead jumps around more than it did, usually goin down way to fast. I did pull a heavy load & run my tank till the fuel light came on & dte = 0 all for the first time recently. It sounds good & runs strong as far as I can tell. Possibly a little more black smoke on acceleration but I could be paranoid. I now have 98,500 miles. Any ideas on what might be goin on?