Rear derailleur noise in big/big combination Every Google search I've done over the past weeks has landed me on this great forum, so thought I would register.
Basically my son gave me his 1996 Scott Apache adventure mountain bike in a box and I've rebuilt it with new bearing, cables etc.

The problem I have is the chain seems to be rubbing on the side cage of the derailleur,  only when it's on the high front and rear cogs.
The chain is two links shorter that the original which was to badly rusted to re-use.
Old chain = 110
New chain = 108
I did rap the new chain around both large cogs and added two which gave me the 108.
Here's a couple of images:
Drive trains with a front derailleur system are designed with to have overlapping gear ratios ranges for each front chain ring to allow you to spend most of your time riding with the chain not stretched to the extremes of your rear cassette.  Riding in your largest gear on both your front and rear is call Cross Chaining.  Cross Chaining puts unnecessary stress on your drive chain system by forcing your chain to work through extreme angles to propel your bike forward.  

Additionally, you'll get a grinding noise when using the most extreme gear combinations as you are forcing your chain to go from front to extreme back.  

Best best to avoid that sound is to practice shifting both your front and rear derailleur.  Starting out, it may feel awkward but your bike and ears will thank you ;-).  

It is worth spending some money at the mechanic to give it a good inspection to make sure everything is in safe working condition as well.  Nothing is worse than having something break when you are riding.