Have you tried re-seating the cpu or even trying the cpu in another system?
If it powers on, but doesn't beep even when there's no RAM or GPU it might be something to do with the CPU (or mobo).
I've seen it happen a few times before & a few times it has been the mobo, other times it's either the way the CPU is seated or the CPU is blown
On one occaision I've seen it happen in a case, then when we powered it up on cardboard it was ok, but in case again it did same. That time it was one of the standoff's on the case that was shorting out the mobo
soon sorted once we realised what it was tho