The things we think of!...
I'm going to venture a guess on this one. I'm not sure it's really your "heart" you're hearing when you're sick, but rather the arteries in your head and neck (carotid etc.) that pulsate in rhythm with your heart. When you're sick, there is a certain amount of congestion in the head and middle ear cavity which serves to enhance the resonance (occlusion effect) and thus you will hear internal sounds you may not have heard before. (Rest assured....even if you don't hear it....it's there and beating ;-)...we hope)
Harriet B. Jacobster, Au.D., CCC-A, FAAA
Board Certified in Audiology
IHAER - Institute for Hearing Arts Education & Rehabilitation, LLC