Q: Why is my iMac beeping instead of starting?

Depending on which model iMac you have and the configuration of the memory currently installed, you may be able to remove a couple of the DIMM's and then swap the positions of the remaining DIMM's to isolate a possibly bad or incorrectly spec'ed DIMM. I suggest doing the following:

1) Refer to iMac: How to remove or install memory to find out how to properly remove the DIMM's and the supported memory configurations for your particular model.

2) Verify that the DIMM's installed are compatible with your system. Each name brand DIMM should have some sort of label providing the specs. Compare the information to the information found in the iMac: Memory specifications and upgrades article.

3) If the memory meets the specifications listed in the document listed above, then try swapping the DIMM's. If you have four DIMM's installed, take out two of the DIMM's and insert the remaining two in the correct slots. While the DIMM's are out check the gold fingers on each one to make sure they are nice and clean. Each system may have different requirements about whether particular slots need to be populated in order for the system to complete the power on self test. If you provide more specific information about your iMac we can provide more detailed assistance.