Problem with F5 heartbeat

I'm thinking you haven't provided enough information here to know for sure...

What are the interfaces connected to? At which speed & duplex? Are they on the same VLAN? Do you KNOW they're on the same VLAN (i.e. can you prove it. If it's a cisco switch then CDP is a big help here)? LLDP is a big help too if it's not a cisco switch.

If you tcpdump on the interfaces do you see traffic inbound from the other node? What about outbound? (Hint, tcpdump at both end simultaneously. You can eliminate a lot of potential problems if you know where the traffic is going missing).

H