I noticed my son's 68 has a loud/annoying clicking sound under the hood.  Normally it purrs like a kitten.  Wondering if this might be his alternator.  Any advice on where to start?  Also, I've ordered some parts from him from a place called Metro parts.  It's been months so I cancelled.  Any advice on the best website to get 68 Mustang parts?  Thanks in advance.  Mac  

Is the sound coming from the engine or elsewhere?  Does it vary with the engine speed?  Is it there all the time or does it go away after the engine warms up?

There are several reputable Mustang parts supppliers on-line.  www.mustangsunlimited.com is one.  I ordered from the a lot when I lived in Atlanta because I could drive to their warehouse and pick the stuff up, avoiding shipping charges.  

Get your parts from National Parts Depot or K.A.R. in Columbus Ohio. NPD is fully stocked and has great customer support!

Without hearing the click I can't diagnose it. The most common click is noisy lifters. This can be from wear, low oil, sludged up engine, etc. Another click can be worn rod bearings. Worn bearings will click constantly, so it might not be that. Again, rod bearings get noisy from wear, abuse, etc. Bad bearings will sometimes sound worse than at other times depending on load, RPMs, even under no load at all. If what you hear is a knock rather than a click it could be worn main bearings, bent rod, etc. If its a hissy click it could be a blown exhaust manifold gasket or cracked exhaust manifold. All of these will have the click increase in speed as the engine revs up. My six will tap if I'm a quart low on oil. The fact that your click comes and goes sounds like an oil pressure problem, but again, I can't tell without hearing it. Bad oil pressure can result from worn oil pump, partually clogged oil pump screen, sludged up engine, low oil, etc.

You don't get clicks from alternators or power steering pumps. You can get a slick from a starter with a broken tooth on the bendix gear, but only when starting.

You need to tell us more about the click!

Harry  

Get a long handled screw driver and try to locate the general vicinity of where the click is coming from.  Now, put the business end of the screw driver in the general area against the engine, and put the handle end up next to your ear.  This acts like a stethoscope and you will be able to hear exactly where the click is coming from.  Generally engine noises come from worn valve lifters.  The next time the engine is a quart low, put in a quart of Marvel Mystery Oil.  
National Parts Depot is undoubtedly the best place to get Mustang parts.  

I'll bet dollars to donuts you hear a click in the valve train. I had the same issue with my 65 and a quart of marvel oil at the next oil change quieted it right down.  National Parts Depot (NPD) is the only place to order classic mustang parts. Their parts are top quality and their customer service is excellent.