Someone doing ping sweeps looking for hosts.
Malware doing its thing.
Background noise is just a fact of life on the internet.
To be RFC compliant people should not block ICMP packets.
Your reply may be disingenuous. There is a case of being protocol compliant and then the recommendations of the RFC documents, or just down right "Because the RFC told you so".
An example of being a specific TYPE of ICMP packet that MUST BE permitted to be RFC compliant would be RFC 2979 - 3.1.1. Path MTU Discovery and ICMP.
However, I don't believe this helps the OP but should be stated anyway in case people start thinking that permitting things like ICMP REDIRECT is a MUST for RFC compliance - where one might not need to accept ICMP REDIRECT packets at all.
There is a rather long list of ICMP TYPES. Usually the (better) rule of thumb is to permit what is 'useful ICMP' for your environment and then rate limit those that you permit.
Just whitelist. Allow known-good, prohibit all else. Sure, according to the RFC's, your coffee machine has to comply with the COFFEE/HTTP Protocol! http://www.ietf.org/rfc/rfc2324.txt
As for PMTUD... Ugh. Get rid of it. I won't bother getting into it, but "Silence on the wire" explains why it is silly.
*note, obviously not being serious about the coffee protocol, but it IS a RFC