Startup Tones and Blinking Lights
The Macs distinctive startup chime is more than an aural brand, it indicates a successful hardware (POST) test. If you dont hear the chime but instead hear a single tone, a series of tones, or see blinking lights, your Mac is indicating a hardware problem. This may be the logic board, bad RAM, the power manager, a video card, etc..
Bad RAM can be a common culprit on newer Macs, a blinking sequence of lights at startup can indicate this condition. Fortunately this is easy to test for: shut down the computer, remove one RAM DIMM at a time (remove in pairs on the Mac Pro or PowerMac G5), then reboot. Repeat for each DIMM in succession until you find the culprit. See your Mac Owners Manual (or Google for instructions) on to remove and install RAM.
If RAM isnt the issue, check the Apple Knowledge Base for information. Unfortunately non-standard startup tones often require a trip to a Mac service center for further diagnosis. These tones are the Check Engine lights of the Macintosh world.
Once the screen turns grey the hardware tests are complete, and the Mac looks for a boot volume. Software now dominates the startup process.
Read more at
http://www.cultofmac.com/50685/how-to-fix-common-mac-startup-problems-macrx/#fu9RsUGAz0ZtVrwq.99