Multiple 403's- Hackers?

multiple 403 error codes and I'm worried

[mttsatx]

The past couple of weeks, I've had lengthy error logs made up of the following:
[Wed Apr 23 14:41:59 2008] [error] [client xx.xx.xx.254] File does not exist: /home/blahblah/public_html/403.shtml

The errors are identical from the same ip and at the same time listed 8 or more times. Then another error from the same ip up to 8 times but two minutes later and all identical. After an hour or so, the process repeats but from a different ip.

I've never had this before and wonder if somebody is trying to hack into my site. Any thoughts or suggestions?
Thanks,
MT