Dell Inspiron 3521 wont start.

Dell Inspiron 3521 18 month old won't start. When i press the start button it does not beep but rather produce a short low pitch noise at a regular intervals.  The sound comes from the speaker. It never reaches any screen signal (logo, icon, bios screen). By luck sometimes it boots and "behaves" absolutely normal. I replaced the memory chips and the problem persists, i checked the performance of the disk in a different computer and it works normal.  What else, mother board, power supply?