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Actually I just Googled backlink audit, and holy hell, what a find!  Someone was definitely deliberately attacking me with negative SEO (looks like a bunch of hacked wordpress sites pointing at us with various keywords).  Thank you for the idea!

Thank you Britney, just to be clear my site isn't about loans at all, it just showed a traffic drop when Google's "Pay Day Loan" algo went live.

Thank you for responding, what specifically would the back link audit entail, do you have a good resource that discusses this?  Google hasn't marked me for any bad links, but the site has been around so long I'm sure some directories and things we were in back in the day which were considered white hat are now black hat etc...

So this is a very old problem, but I'm finally getting around to trying to figure it out.  My site experienced a dramatic organic traffic drop from Google (-40%) on May 17th 2014.  It then drops another 30% on May 19th 2014.  See graphs.  According to Moz these two dates correlate with Payday Loan 2.0 and Panda 4.0.

Panda makes complete sense, as this site (www.ausedcar.com) has a large amount of content that is syndicated across other sites (used car inventory is essentially the same everywhere on the Internet).  Payday Loan on the other hand, which seems to be the primary traffic drop doesn't make any sense at all.  Is it possible I started getting hit by Panda on the 17th and then it completed on the 19th?  I know the dates for algorithm changes are not perfect.

Next, assuming it is Panda, what are some things you guys have done to help with this?  As I mentioned this content is duplicated all over the Internet, so it seems like Google arbitrarily picks winners and losers (my site is twenty years old!).  I know I need unique content, but not sure how exactly to do that besides rewriting words so it doesn't appear duplicate.