I have a 94 mobile home and we have an air unit outside and the furnace in the house, we just flip the switch from heat to cool and it has began to make this sound that almost sounds like there is someone taking a shower in the guest bathroom, there is no banging or ratteling just that, I don't know what could be the problem or if it is safe to run with the noise, there is no smell of anything burning. Just the noise, do you know what the problem could be or is there something I can look for myself to fix it. Thank you,

Sounds like you are hearing the refrigerant moving in the copper lines from the outside A/.C unit to the indoor coil at the furnace. If the unit is working and the noise does not bother you it should be OK but if the noise gets louder the outside unit could be starting to have a problem. Have your unit serviced in the spring before the real hot weather.