Late last summer I had what I think was a similar issue, a ping/clicking while under load. The ping was almost not noticeable when idling. When driving along even a slight grade, or starting off, I noticed the noise. I took to the shop and found that several of the exhaust manifold bolts has broken off. Resulting in escaping gas from around the manifold and the head. The broken bolts were removed and replaced in about 3 hours. Luckily, there was no need to drill out and insert new threads. this will be a more expensive job. If this is an issue, get this looked at sooner than later, there is a possibility that the manifold could also warp and there will be a added cost for a new manifold or a shaving down to straighten the manifold. Best of luck