2001 Honda Accord Car overheats on hills

My car overheats on hills when its hot and the air cond is full on. When I turn on heater or turn onto level street, the temp goes back down to normal. Honda says my thermostat is good and there are no leaks in radiator. Radiator shop says my rad is plugged and needs replacement. They only visually examined it. Other symptoms. When I return home in my garage sometimes I hear 20 sec gurgling sound from engine. There's also a very low freq hum for about 2 min. And an acrid sweet smell pervades the garage.I think it's coolant. There's no spillage on the ground or smoke from tailpipe. Dealer wants to conduct leak teat on heads. Any thoughts? Thanks