The last gas question

[Tonavi]

I know octane and gas questions have been covered elsewhere on the forum, but in all of those threads there isn't a clear answer on the subject.

I bought my hatch in August 2007, and from the day I drove it home on the first dealer-bought tank of gas, noticed what sounded like a valvetrain rattle or something like that. I continued to put in regular 87 octane gas and the car kept rattling, and it was very easy to hear with the windows down.

After talking to a mechanical engineer, I tried using the 89 octane stuff to quiet things down, and the car runs noticeably smoother and is silent. My first tank of 89 was at 1600 miles, and now I'm at almost 21,000.

I was reading a Fit forum on FE when I read a post that was about how using higher octane gas could slowly wreck your motor over time by burning too hot and causing timing problems. That's when I went on Google and started researching. Let me note that switching from 87 to 89 resulted in no gain in mileage that probably wasn't related to the break in period. I get between 38 and 44 normally.

Is it true that using +2 octane gas causes carbon buildup and such? If that's the case, wouldn't running Seafoam every 10,000 miles keep things clean and more efficient? If anything, premium is a waste of money, but the extra $0.03 seems to make a difference in the drive.