car runs fine, but engine sounds scratchy
 

I Just Bought An 84 300d A Week Ago. Yesterday I Filled It Up For The First Time Since Buying It, And Now The Engine Sounds Really Rough And Loud. It Still Runs Good, It Just Sounds Like Crap. I'm Afraid I Either Got Bad Fuel Or Accidentally Used Off-highway Diesel. What Can I Do?

I don't own a diesel and have heard from other people that these products are supposed to be great:

http://www.etyparts.com/lm%202002%20pic%20+%20desc.htm

http://www.etyparts.com/lm%202005%20+%20pics.htm

i just used some STP fuel treatment, and that didn't seem to help any. when i get the car up to speed it sounds fine, but at low speeds when i ease my foot off the gas, the engine makes a scratchy, abrasive sound. any ideas? should i take it in?