Hearing unusual noises from the engine compartment when going up steep hills, the first thing I think of is engine pinging. That’s the condition when pinging will be noticed, if the engine is on the verge of pinging. Of course before doing anything else, once everything cools down, check the oil and transmission fluid and the coolant levels to make sure they are good. For the coolant it isn’t sufficient to just look at the plastic bottle, you have to remove the cap and peer into the radiator too.
Pinging can be caused when the spark plugs are left in the engine too many miles and the gap opens up. What’s the routine maintenance situation? Any engine stuff lagging behind the schedule the manufacturer recommends?