Rattling noise at low speed when the engine is hot

Its from the engine.. some irritating noise at low speed or when climbing hills..

Ok, If it sounds something like a diesel engine when going up a hill, this is likely 'detonation'. In this condition the fuel in one or more cylinders is getting hot and burning before the spark plug ignites. Typically you would see a check engine light though. Next I would consider the timing chain tensioner and possibly oil sludge buildup. Since it happens when driving low speed, when oil pressure is low, and when driving at heavy load (uphill) when oil pressure is needed most, a failing timing chain tensioner may not be able to keep up, or engine sludge might be preventing sufficient oil delivery to metal-metal contact points like lifters or connecting rods. If your check engine light is on, I encourage you to have it checked (free at auto parts stores). If not, have this inspected at your earliest convenience, at least to pinpoint the issue and severity.